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The reaction of 2‐arylidenecyclohexanones 1 with dithiocarbamic acid gave three of the four possible diastereomers of 4‐aryl‐4a,5,6,7,8,8a‐hexahydro‐8a‐hydroxy‐4H‐3,1‐benzothiazine‐2(1H)‐thiones 2–4. The isomeric composition of the reaction products was found to depend on the quantity of hydrochloric acid used as catalyst. 1H‐NMR studies showed that the preferred conformation of the cis isomers 2 and 4 is controlled by the bulky 4‐aryl group, which always occupies the energetically more favourable quasiequatorial position. Dehydration of 2–4 afforded the corresponding 4‐aryl‐tetrahydro‐4H‐3,1‐benzothiazine‐2(1H)‐thiones 5 and 6. The orientation of the dehydration reactions depends on the configuration of the starting compounds 2–4 and the reaction conditions used.

Starting from the readily available keto ester 1, through intermediates 3, 6a, 6b, 6j, and 6k, a stereospecific total synthesis of disubstituted alloyohimbanes of type 7 was accomplished. Alloyohimbine (8m), «-yohimbine (8n), and the other two possible stereoisomers (8h and 8i) were also prepared. In the course of these transformations, the first example of imino ether-enamine tautomerism, neighboring-group participation in the hydrolysis of compounds 6a and 6b, and a Knoevenagel condensation under extremely mild experimental conditions were observed and studied.

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