A new synthesis of 1 by selective deacetylation of 2 is reported. The acylation step implied in earlier syntheses of 1 and related compounds is exemplified by direct acylations of 3 to give 4a and 46. Several new 2-acylidene-3-(2,4-dibromophenyl)-5-phenyI-2,3-dihydro-1,3,4-thiadiazoles (11) and thioacylidene analogues (12) are described. The crystal structures of l l a , l l b , l l c , and l l d reveal a hypervalent interaction, ca. 2.45-2.7 A long, between the sulfur and carbonyl oxygen atoms. The dibromophenyl ring is nearly perpendicular to the thiadiazole ring plane in the crystal structures, and NMR data suggest that this conformation is maintained in solution. Molecular mechanics calculations show that the S...O interaction need only be a few kilocalories in order to_ stabilize the observed acylidene side chain configuration over other pcssible isomers. Crystals of l l a are triclinic, P1, with cell dimensions a = 12.937(2), b = 13.429(2), c = 13.489(2) A, a = 60.14(1)", P = 74.59 (1)

On p. 569, the structural formula for 15a and b should have a positive charge on N3:

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