Synthesis and ene reactions of di-(−)-menthyl diazenedicarboxylate

The preparation di-(−)-menthyl diazendicarboxylate 3 is described. Reaction of hydrazine with excess (−)-menthyl chloroformate 1 afforded dimenthyl diazanedicarboxylate 2 which was then oxidized to the azo-enophile 3. The azo-ene reactions of 3 with the alkenes trans-3-hexene 4, trans-4-octene 6, cyclohexene 8 and cyclopentene 10 were carried out using the Lewis acid catalyst tin tetrachloride. Trans-3-hexene 4 and trans-4-octene 6 afforded the ene adducts 5 (80%) and 7 (70%) with a diastereomeric excess of 2.5:1 and 2:1 respectively. Use of cyclohexene 8 and cyclopentene 10 afforded the ene adducts 9 and 11 in 81% and 88% yield, however, no diastereomeric excess was observed. Finally use of conjugated aromatic acyclic alkenes 12 and 14 afforded the chlorides 13 and 15 in 93% and 56% yield respectively. In these latter cases an ionic addition proceeded rather than a pericyclic ene reaction. Attempted removal of the chiral menthyl ester auxiliary from the ene adduct 5 proved difficult.

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