The parent boroxine (cyclo-(HBO)) is prepared in small quantities as a low pressure gas by high temperature reaction of water and elemental boron or reaction of various boranes (BH or BH) with O.  It is thermodynamically unstable with respect to disproportionation to diborane and boron oxide.  Some reactivity studies and an IR spectrum are reported, but it is otherwise not well characterized.
As discovered in the 1930s, substituted boroxines (cyclo-(RBO), R = alkyl or aryl) are generally produced from their corresponding boronic acids by dehydration. This dehydration can be done either by a drying agent or by heating under a high vacuum.  
Trimethylboroxine can be synthesized by reacting carbon monoxide with diborane (BH) and lithium borohydride (LiBH) as a catalyst (or reaction of borane–tetrahydrofuran or borane–(dimethyl sulfide) in the presence of sodium borohydride):