described below is the preparation of cyclic peptides according to the present one - step synthesis and purification method , including the preferred embodiments of the invention , such as vasopressin and derivatives or analogs of vasopressin , particularly desmopressin ( ddavp ). without limiting the invention thereto , the method according of the invention will be explained in greater detail by means of the following examples . the immobilized open - chain nonapeptide derivative i was prepared by the merrifield synthesis with the appropriate n - tert . - butyloxycarbonyl - protected ( boc ) amino acids and with mbha ( 4 - methyl benzhydrylamine ) resin ( novabiochem , laufelingen , ch ), in a way similar to that described by krchnak v . and zaoral , m ., 1979 , coll . czech . chem . comm ., 44 : 1173 - 1178 , except that the thiol groups were protected by p - methoxybenzyl and the phenolic hydroxyl group in tyrosine by 2 - bromobenzyloxycarbonyl . boc - cys ( p - methoxybenzyl )- oh and boc - tyr ( 2 - bromo - benzyloxycarbonyl )- oh were obtained from novabiochem and other commercial sources ; β -( p - methoxybenzyl ) mercapto - propionic acid was synthesized in analogy to the synthesis of s - benzyl - cysteine ; cf . acta chem . scand 13 , 383 ( 1959 ). the immobilized nonapeptide ( i ) was released by treatment of 2 g resin with a mixture of 10 ml hf , 1 . 0 ml of anisole and 1 . 0 ml of ethyl methyl sulfide and the crude deprotected nonapeptide derivative β - mercapto - propionyl - tyr - phe - gln - asn - cys - pro - d - arg - gly - nh 2 ( ii ) ( seq id no : 1 ) was dissolved in 24 ml 10 % hoac / h 2 o . 1 ml of this solution was diluted with 10 % hoac / h 2 o to a final volume of 53 ml . to this solution was added dropwise with stirring and at ambient temperature 426 μl of a 0 . 1 m solution of i 2 in 95 % ethanol over a period of 90 min . stirring was continued overnight and the slightly yellow solution ( a ) was divided into two equal parts . one - step procedure . one half of solution ( a ) above was loaded on a column packed with s sepharose ® ff cross - linked agarose cation exchanger ( pharmacia ) equilibrated with 5 % acoh and eluted with 0 , 08 m hoac / nh 4 oac buffer , ph 4 . 1 . elution was monitored at 280 nm and each fraction ( 2 . 5 ml ) assayed by hplc . fractions containing pure ddavp were pooled and lyophilized . white fluffy powder ; yield 18 mg ( 76 %), ddavp - content 94 . 5 % by hplc . two - step procedure . the other half of solution ( a ) was cooled in an ice bath and decolorized by titration with 1 m aqueous sodium dithionite . the resulting solution was passed through a column packed with 2 g of licroprep ® rp - 18 ( c 18 - reversed phase column ; merck , darmstadt ) and prewashed with h 2 o . after removing salts by washing with 30 ml h 2 o ddavp was eluted with 15 ml of etoh / hoac / h 2 o ( 85 : 5 : 15 , v / v ). 15 ml h 2 o was added , etoh evaporated under vacuum and the resulting solution purified on the sepharose ff cation exchanger as described above . white fluffy powder , 17 . 5 mg ( 74 %; purity 94 % by hplc ). comparison of the one - step procedure according to the invention with the two - step procedure shows that the former provides the product in higher yield , shorter time and essentially the same purity . the thus obtained product may be further purified by , e . g ., gel filtration , reversed - phase chromatography , and the like . the immobilized open - chain nonapeptide derivative iii was prepared by conventional solid phase technique using fmoc -( fluorenylmethyloxycarbonyl ; cf . j . chem . soc . perkin i 1981 , 536 ; 538 ) and t - butyl - protected amino acids obtainable from commercial sources . the immobilized compound ( iii ) was hydrolytically cleaved from the resin ( tentagel ® s - ram ; rapp polymere , tubingen , germany ) and the protecting groups partially removed by treatment with a mixture of 19 ml tfa , 0 . 5 ml anisole and 0 . 5 ml of ethane dithiol at ambient temperature . the resin was removed by filtration , the filtrate evaporated and the peptide triturated with diethyl ether . the crude peptide derivative β - acetamido - methylmercaptopropionyl - tyr - phe - gln - asn - cys ( s - acetamidomethyl )- pro - d - arg - gly - nh 2 ( iv ) ( seq id no : 2 ) was obtained as a white powder by filtration . yield 251 mg ( 94 %; fab - ms ( m + h ) + = 1212 ). compound iv ( 53 mg ) was dissolved in 53 ml 10 % ( v / v ) aqueous acetic acid . during 90 min 0 . 0426 mmol of iodine in 426 μl ethanol was added dropwise to the peptide solution while stirring and temperature being kept at 20 ° c . stirring was continued overnight . solution ( b ) with a yellowish tinge was divided in two equal volumes . one - step procedure . by following the procedure in example 1 , one half of solution ( b ) was loaded on s sepharose ® ff cation exchange resin and purified as described above . fractions containing pure ddavp were pooled and lyophilized . white fluffy powder , yield 15 . 1 mg ( 64 %), ddavp - content 94 . 5 % ( hplc ). two - step procedure . the other half of solution ( b ) was treated as described in the corresponding part of example 1 . yield of ddavp 12 mg ( 51 %), ddavp - content 94 , 0 % ( hplc ). comparison of the one - step procedure according to the invention with the two - step procedure shows that the former again provided the product in higher yield , shorter time and essentially the same purity . a route corresponding to that described in example 2 was followed , except for trityl being used as s - protecting group . compound v was decoupled by treatment with trifluoroacetic acid / scavenger ( 19 ml tfa , 0 . 5 ml anisole , 0 . 5 ml ethane dithiol ). after evaporation , trituration and filtration of the peptide , the resulting crude β - mercaptopropionyl - tyr - phe - gln - asn - cys - pro - d - arg - gly - nh 2 ( vi ) ( seq id no : 1 ) was dissolved in 274 ml 10 % hoac / h 2 o . 53 ml of this solution was oxidized with iodine under the conditions described in example 2 . the solution containing the reaction products was again divided into parts of equal volume which were subjected to the one - step and two - step i 2 - oxidation / purification procedure , respectively . one - step procedure . ddavp was obtained as a white fluffy powder . yield 17 . 0 mg ( 72 . 1 %), purity 92 . 3 % ( hplc ). tentative preparation of desmopressin ( ddavp ) from β - mercaptopropionyl - tyr - phe - gln - asn - cys - pro - d - arg - gly - nh 2 ( vi ) ( seq id no : 1 ) obtained by homogeneous phase synthesis a solution of crude compound vi obtained by the method of zaoral , et al . ( swedish patent no . 353 530 ) was oxidised with iodine 10 % aqueous acoh as described in example 2 . the resulting solution containing iodine was chromatographed on s sepharose ® ff cation exchange resin as described above . however , the isolation of pure ddavp was not possible by the one - step procedure due to the high sodium ion content of the reaction mixture . this high sodium ion concentration is attributed to the deprotection step using na / nh 3 . the immobilized open - chain nonapeptide derivative vii was prepared by conventional solid phase technique in analogy to the synthesis of desmopressin ( ddavp ; example 2 ), using fmoc ( fluorenylmethyloxycarbonyl ; cf . j . chem . soc . perkin i 1981 536 , 538 ) and t - butyl - protected amino acids obtainable from commercial sources , except for fmoc - d - tyr ( et ) oh which was prepared by fmoc protecting ( biopolymers , 1983 , 22 : 2157 - 2162 ) of h - d - tyr ( et ) oh ( bachem ag , ch ). the immobilized compound vii was cleaved from resin and the protecting groups partially removed by treatment with a mixture of tfa / water ( 95 : 5 ) for 2 h at room temperature . the resulting β - acetamido - methyl - mercaptopropionyl - d - tyr ( et )- ile - thr - asn - cys ( s - acetamidomethyl )- pro - orn - gly - nh 2 ( viii ) was triturated with ether , collected by filtration , washed with ether and obtained in form of white powder . overall yield 70 %. characterized by fab - ms ; ( m + m + = 994 ). 48 . 3 mg viii was oxidized with iodine by the method described above and the resulting solution divided into two equal parts of which one was purified by the one - step method . yield of atosiban ( ix ) 12 mg ( 57 %), purity 90 % hplc ). while the invention has been fully disclosed hereinabove , it is within the skill of those in the art to modify those methods , reagents and reaction conditions described herein to obtain similar results . such modifications are within the spirit and scope of the present disclosure . for instance , the foregoing methods may be applied to oxytocin and its analogs with the expectation that similar advantageous results will occur in terms of both yield and purity . __________________________________________________________________________ # sequence listing - ( 1 ) general information :- ( iii ) number of sequences : 6 - ( 2 ) information for seq id no : 1 :- ( i ) sequence characteristics : ( a ) length : 6 amino - # acids ( b ) type : amino aci - # d ( c ) topology : linear - ( ii ) molecule type : peptide - ( v ) fragment type : internal fragment - ( xi ) sequence description : seq id no : 1 :- tyr phe gln asn cys pro1 5 - ( 2 ) information for seq id no : 2 :- ( i ) sequence characteristics : ( a ) length : 4 amino - # acids ( b ) type : amino aci - # d ( c ) topology : linear - ( ii ) molecule type : peptide - ( v ) fragment type : internal fragment - ( xi ) sequence description : seq id no : 2 :- tyr phe gln asn - ( 2 ) information for seq id no : 3 :- ( i ) sequence characteristics : ( a ) length : 9 amino - # acids ( b ) type : amino aci - # d ( c ) topology : circular - ( ii ) molecule type : peptide - ( ix ) feature :# cys at position 1 linked to cys at # 6 by a disulfide moiety - ( xi ) sequence description : seq id no : 3 :- cys tyr phe gln asn cys pro lys gly1 5 - ( 2 ) information for seq id no : 4 :- ( i ) sequence characteristics : ( a ) length : 12 amin - # o acids ( b ) type : amino aci - # d ( c ) topology : circular - ( ii ) molecule type : peptide - ( ix ) feature :# cys at position 4 linked to cys at # 9 by a disulfide moiety - ( xi ) sequence description : seq id no : 4 :- gly gly gly cys tyr phe gln asn cys pro ly - # s gly # 10 - ( 2 ) information for seq id no : 5 :- ( i ) sequence characteristics : ( a ) length : 8 amino - # acids ( b ) type : amino aci - # d ( c ) topology : linear - ( ii ) molecule type : peptide - ( v ) fragment type : internal fragment - ( xi ) sequence description : seq id no : 5 :- phe ile gln asn cys pro xaa gly1 5 - ( 2 ) information for seq id no : 6 :- ( i ) sequence characteristics : ( a ) length : 8 amino - # acids ( b ) type : amino aci - # d ( c ) topology : linear - ( ii ) molecule type : peptide - ( v ) fragment type : internal fragment - ( xi ) sequence description : seq id no : 6 :- phe ile xaa asn cys pro xaa gly 1 5__________________________________________________________________________