in the following section one preferred embodiment is described by way of example to illustrate the process of this invention . however , these are not intended in any way to limit the scope of the present invention . to a solution of p - nitrobenzyl ( 5r , 6s ) p - nitrobenzyl - 6 [( 1r )- 1 - hydroxyethyl ]- 1 - azabicyclo [ 3 . 2 . 0 ]- heptan - 3 , 7 - dione - 2 - carboxylate ( 20 . 0 g , 57 mmol ) in a mixture of tetrahydrofuran and 1 , 3 - dimethyl - 3 , 4 , 5 , 6 - tetrahydro ( 2h ) pyrimidinone ( 160 ml ; 1 : 1 v / v ) was added diisopropylamine ( 7 . 0 g , 69 mmol ) at − 25 to − 30 ° c . followed by diphenylchlorophosphate ( 17 . 0 g , 63 mmol ). the mixture was stirred for 40 - 45 min . at − 10 to − 15 ° c . and used in the next step without isolation of the enol phosphate intermediate . the reaction mixture from step i was cooled to − 75 ° c . a solution of cysteamine ( 2 - aminoethanethiol ) hydrochloride ( 7 . 2 g , 63 mmol ) in 1 , 3 - dimethyl - 3 , 4 , 5 , 6 - tetrahydro ( 2h ) pyrimidinone ( 70 ml ) and diisopropylamine ( 7 . 0 g , 69 mmol ) was added at − 75 to − 50 ° c . in 10 min . the reaction mixture thus obtained was further stirred at − 40 ° c . to − 45 ° c . for about 1 hr and was used as such without its isolation . the reaction mixture from step ii was cooled to − 45 ° c ., and diisopropylamine ( 8 . 7 g , 86 mmol ) was added followed by benzylformimidate hydrochloride ( 12 . 8 g , 74 . 6 mmol ). the resulting mixture was stirred at − 45 to − 40 ° c . for about 30 minutes and then at − 20 to − 15 ° c . for about 1 hour 30 minutes . tetrahydrofuran ( 200 ml ) was added at − 20 to − 15 ° c . and diisopropylamine salts were filtered off from the reaction mixture . the filtrate contained the n - formamidoyl thienamycin pnb ester . the filtrate from step iii containing pnb - n - formimidoyl thienamycin was poured into a mixture of water ( 400 ml ), n - methylmorpholine ( 18 g ), isopropyl alcohol ( 200 ml ) at 0 - 5 ° c . and at ph of about 7 . 0 . the reaction mixture was then hydrogenated over at 3 - 4 kg of hydrogen pressure at 5 - 10 ° c . the reaction mixture was filtered , extracted with methylene chloride ( 2 × 300 ml ) and the aqueous layer was separated . the aqueous layer was concentrated to 200 ml by distillation under reduced pressure . the concentrated solution was purified by dianion chromatography using water as an eluent . the fractions containing the desired product were combined and concentrated either under reduced pressure or using plain membrane type reverse osmosis techniques to a volume of 50 ml . the concentrated solution was then cooled to 0 - 5 ° c . and isopropyl alcohol ( 25 ml ) was added to it . it was further stirred for 40 - 45 min at the same temperature . another lot of isopropyl alcohol ( 25 ml ) was added and stirring continued for about 1 hour at 0 - 5 ° c . the crystalline precipitate obtained was filtered , washed with isopropyl alcohol and acetone ( 2 × 10 ml ) and dried to yield 4 . 0 g of crystalline n - formimidoyl thienamycin . x - ray diffraction pattern ( fig1 ) shows peaks characteristic of crystalline form of imipenem monohydrate as obtained per u . s . pat . no . 4 , 260 , 543 ; purity by hplc = 99 . 23 %. while the present invention has been described in terms of its specific embodiments , certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present invention .