3-Alkynoates, rearrangements with

Polysubstituted 1,3-oxazolidines were prepared in a one-pot diversity oriented four-component reaction (4-MCR), comprising two linked domino processes. Thus, domino synthesis of enol ethers 9 was followed by a sequential amine addition-cyclization sequence [74]. While strong microwave irradiation (900 W) of silica-gel absorbed conjugated alkynoates 9 and amines afforded tetrasubstituted pyrroles (via the skeletal rearrangement of 1,3-oxazolidines, see Sect. 2.1 and Scheme 5) [24], the use of milder microwave conditions (160 W power, 90 min) furnished 1,3-oxazolidines. Under these mild conditions the 1,3-oxazolidines did not rearrange to pyrroles and with respect to diastereoselectivity, the 1,3-oxazolidines were obtained as mixtures of syn/anti isomers. Overall, the formation of one C-C bond, one C-0 bond, two C - N bonds and a ring in this MCR required less than 3 hours and utilized simple and commercially available reagents (Scheme 26). 

The reactions of alkynes, alkynoic esters, ynamines, and dehydrobenzene with l,2-dithiole-3-thiones (2b), and occasionally l,2-dithiol-3-imines (12c), has been extensively studied <82AHC(3i)63>. The initial products for thiones are 3-thioacylmethylene-l,3-dithioles or thiazoles (111), which can react further to form di- or tri- adducts (113) and (114) respectively. Other rearrangements can give trithiapentalenes (6). Thials, formed from reactions of 5-unsubstituted-l,2-dithiole-3-thiones, can dimerize to products (112) with loss of sulfur (Scheme 14). Similar types of reactions have been observed with l,2-dithiole-3-thiones (2b) reacting with alkenes and ketenes, especially under... 

Treatment of 26 with benzylamine under microwave irradiation led to the formation of 1,3-oxazolidine 27 which subsequently rearranged to give the pyrrole 28. The serendipitous discovery of the latter reaction provides a novel domino process for the formation of pyrroles from acyclic enol-protected propargyl alcohols which are also be derived from a domino process, the double condensation of alkynoates with aldehydes <04JA8390>. 

In 2009, Tan and coworkers reported that the chiral bicyclic guanidine (S,S)-20 was able to catalyze the enantioselective isomerization of 3-alkynoates to chiral allenoates in what (formally) constitutes a 1,3-hydrogen sigmatropic rearrangement (Scheme 40.29) [33). The reaction took place in hexane at low temperature with high enantioselectivities, but conversions were not complete. 

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