E-Erythro vicinal diamine

Since 3,6-dihydrothiazine 1-imines exist in conformations having a quasi-axial S—N bond, these imines are properly disposed stereoelectronically for such a pericyclic process. This reaction is essentially irreversible, similar to the equilibrium in a [2,3]-sigmatropic rearrangement of allylic sulfilimines (55), which lies to the side of the sulfenamide (56) (Scheme 11). There is a clean transfer of stereochemistry from C-6 of the dihydrothiazine (52) to C-4 of the thiadiazolidine (53), leading to the (E)-erythro-vicinal diamine derivative (54). In a similar fashion, thermolysis of the epimeric imine (57) gives the thiadiazolidine (58), which can be converted into the Z)-threo-vicinal diamine... 

A similar situation exists with sulfur epimer 61, prepared from (E,Z)-2,4-hexadiene, which is unreactive toward phenylmagnesium bromide and gives E-erythro vicinal diamine 59 in only poor yield with methy-llithium. Inspection of the conformations of the adducts 61A and 58A indicates that the path of the attacking nucleophile in the former case is more sterically crowded than in the latter. 

Since the Diels-Alder cycloadditions used to prepare various dihy-drothiazine imines generally produced mixtures of sulfur epimers, it was desirable to find a procedure to also convert the unreactive adducts to the vicinal diamines. It was discovered that adducts 60 rearranged thermally by a novel [2,3]-sigmatropic process to afford thiadiazolidines 62 stereoselectively (Scheme 1-XV). Reduction of 62 with sodium borohy-dride yielded the -threo vicinal diamine derivative 57. Similarly, unreactive adducts 61 cleanly gave thiadiazolidines 63 on heating, which could be converted to E-erythro vicinal diamines 59. This methodology thus allows both sulfur epimers from ( , )-2,4-hexadiene to be converted to the -threo product, and the epimers from ( ,Z)-2,4-hexadiene to be used to prepare the -erythro series of compounds. 

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