4-Deuterio-5-phenylthiazole

In the dyad, because of the symmetry of the 3-phenylazetine ring in BC-53 (Scheme 29), insertion of sulfur between ring positions C-l and C-4 or CC-1 and C-2 leads to the same compound, 5-phenylthiazole (53). Similarly, insertion of a sulfur atom between N3 and C4 or N3 and C2 leads to 4-phenylisothiazole (55). Accordingly, because of this symmetry, only a dyad results. This symmetry is removed, however, in the case of 2-deuterio-5-phenylthiazole (53-2d). Thus, irradiation of 53-2d resulted in the formation of three isomeric products (Scheme 30), viz., 4-deuterio-5-phenylthiazole (53-4d), 5-deuterio-4-phenylisothiazole (55-5d), and 3-deuterio-4-phenylisothiazole (55-3d). D euterium 1 abelling h as t hus e xpanded t he d yad i nto a tetrad. These results are entirely consistent with the electrocyclic ring closure - heteroatom migration mechanism shown in Scheme 31 <94JA2292>. 

In benzene solution 56 phototransposes (Scheme 38) only by the electrocyclic ring closure-heteroatom migration pathway to yield 51, 52 and 54 in 2%, 15%, and 5% yields respectively. In the presence of TEA, however, the major product (Scheme 39) of the photoreaction of 56 is 5-phenylthiazole (53) formed in 14% yields, with 52 and 54 formed in 4% and 5% yields respectively. 4-Deuterio-5-phenylisothiazole (56-4d) also phototransposed (Scheme 40) to 4-deuterio-5-phenylthiazole (53-4d), confirming that this product was formed by the N-2-C-3 interchange pathway, and 52-5d and 54-4d as expected for products formed by the electrocyclic ring closure-heteroatom migration mechanistic pathway. 

Thus upon prolonged irradiation the initially formed 2-deuterio-4-phenylthiazole 51-2d transposes to 5-deuterio-3-phenylisothiazole 47-5d without additional deuterium incorporation [36]. 

Later work, shown in Scheme 12, confirmed that 3-phenyKsothiazole 13 is formed with deuterium incorporation at C4 upon irradiation of either 2-phenylthiazole 18 or 5-phenylisothiazole 15 in benzene/ DjO but revealed that 13 is formed with approximately twice the extent of deuterium incorporation from the latter reactant. Furthermore, it was also shown that 4-phenylthiazole 17 undergoes photodeuteration at Cj when irradiated in benzene/DjO more rapidly than it transposes to 3-phenylisothiazole 13. Upon further irradiation, the initially formed 2-deuterio-4-phenylthiazole 17-2 transposes to 5-deuterio-3-phenyl-isothiazole 13-5d without additional incorporation of deuterium. 

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