ASYMMETRIC AZO-ENE REACTIONS USING THE CHIRAL AZO-ENOPHILE DI-(-)-(1R,2S)-2-PHENYL-1-CYCLOHEXYL DIAZENEDICARBOXYLATE

The preparation of di-(-)-( 1R,2S)-2-phenyl-1-cyclohexyl diazenedicarb oxylate 4 is described. Reaction of (1R,2S)-2-phenyl-1-cyclohexanol 1 with excess phosgene in the presence of quinoline afforded chloroforma te 2 which was treated directly with hydrazine monohydrate (0.5 equiv. ) to afford di-(-)-( 1R,2S)-2-phenyl-1-cyclohexyl diazanedicarboxylate 3. Oxidization of 3 to the azo-enophile 4 was then readily effected i n high yield using N-bromosuccinimide and pyridine. The azo-ene reacti ons of 4 with the alkenes cyclohexene 5, cyclopentene 6, trans-3-hexen e 7 and trans-4-octene 8 were carried out using the Lewis acid tin(IV) chloride. Use of cyclohexene 5 afforded the ene adduct 9 in 80% yield with a diastereomeric excess of >97:3 whilst the use of cyclopentene 6, trans-3-hexene 7 and trans-4-octene 8 afforded the ene adducts 10 ( 77%), 11 (71%) and 12 (92%) with a diastereomeric excess of 86:14 in e ach case. Use of the conjugated aromatic acyclic alkene 13 afforded th e product of an ionic addition, namely, chloride 14 in 57% yield. Clea vage of the N-N bond of the ene adduct 9 was effected using lithium in liquid ammonia affording the carbamate 16 in moderate yield. (C) 1998 Elsevier Science Ltd. All rights reserved.