Diphenylacetylene from the Base‐Catalyzed Fragmentation of 3,6‐Dihydro‐5,6‐Diphenyl‐3‐Tosyl‐1,3,4‐Oxadiazin‐2‐One

Abstract

Treatment of 3,6-dihydm-5,6-diphenyl-3-tosyl-I , 3,4-oxadiazin-2 ter than 70% yield.In the context of our work on 3-pyrazolidinones (1) and the related 3-oxopyrazolidinium ylides (2),' we had been interested in tetrahydro-1,3,4-oxadiazin-2-ones (3) which differ from 3-pyrazolidinones only by the presence of an oxygen in the ring. To our knowledge, the investigations of Rosenblum and his group2 remain the only report in that area. In both cases, it is likely that the cyclic a-carbonylazo intermediates,*-3 resulting from the oxidation of l a (R = R' = Ph)*.3 and of 3a (R = R' = Ph)? undergo extrusion of nitrogen and of either carbon monoxide or carbon dioxide, respectively, to give trans-stilbene. On the other hand, it is interesting to note that of the unsaturated analogues 4 and 6, only 4 undergoes a similar oxidativefragrnentation, albeit in very poor yield (14% yield of 6 b) cyclopentadiene 9 m t~l a n ) ;~ indeed, Carpinos had reported the formation of aphenylcinnamic acids (mixture of isomers) from the reaction of 6 with base, evidently by base-induced ring opening of 7; com--one with sodium hydride in THF afirds diphenylacetylene in betpound 7 is apparently sufficiently stable to be intercepted as the Diels-Alder adduct with cyclopentadiene.6In the course of our attempts to devise a preparation of 3s (R = R' = Ph) which would provide both stereoisomers and would not involve the use of sealed tube reactions,2 we synthesized a number of derivatives of 5,6-diphenyl-3,6-dihydro-1,3,4-oxadiazin-2-one (lo), including 3,6-dihydro-5,6-diphenyl-3-tosyl-1,3,4-oxadiazin-2-one (11). It was anticipated that 11 should undergo base-catalyzed 1 ,Celirnination to intermediate 5 which should then fragment to nitrogen, carbon dioxide and diphenylacetylene. Thus, it was gratifying to find that the reaction of 11 with sodium hydride in tetrahydrofuran (THF) at reflux smoothly and cleanly gave tolan in better than 10 11 70% yield. In contrast to 4, which was prepared by treatment of 10 with two equivalents of triphenylmethide, compound 11 was readily obtained as a pale tan solid, mp. 156-158", in 70% yield by treatment of 5,6-diphenyl-3,6-dihydro-1,3,4-oxadiazin-2-one (10) with sodium hydride in THF, followed by the addition of tosyl chloride. Diphenylacetylene could also be prepared directly from 10 in 65% yield by treatment of the tosylation reaction mixture with a second equivalent of sodium hydride.The facile transformation of the 1,3,4-oxadiazin-2-one ring to a triple bond in synthetically acceptable yields suggests that these compounds may be useful as masked alkynes and thus may prove to be valuable, especially in macrocyclic chemistry, since the required a-ketols, precursors of the 1,3,4-oxadiazin-2-ones may be obtained by the acyloin and the benzoin condensations.7-577 -Acknowledgements.-The support of this work by the Department of Chemistry is hereby gratefully acknowledged. Aberdeen Allen also thankful for the award of a Teaching Fellowship.

The Direct Conversion of α-Hydroxyketones to Alkynes

“…We concluded that, if we could develop a direct conversion of acyloins into alkynes, a highly flexible two-step approach for the fragment coupling synthesis of internal alkynes would result. ,, We furthermore hypothesized that achieving this target reaction would require us to couple the alkyne formation to the formation of highly stabilized species, in order to ensure an overall favorable energetic profile. Indeed, the groups of Rosenblum, Marshall Wilson, and Anselme have reported the formation of tolane 1a from substrate 3 , which proceeds through the base induced formation of intermediate 4 and a subsequent thermal fragmentation leading to nitrogen and carbon dioxide as byproducts (Scheme B). Although in these studies substrate 3 was synthesized through different approaches, we wondered if it would be possible to generate this species from benzoin 2a by treatment with tosylhydrazine and a suitable carbonate-derived electrophile such as carbonyl diimidazole (CDI) …”