Butanolides and Butenolides by Intramolecular Ene-Reaction during Thermolysis of Propargyl Propiolates

Article abstract of DOI:10.1002/hlca.19860690730

Gas-phase flow thermilysis of 2-butynyl propynoate (1) and 2-propynyl 2-butynoate (2) at 550 deg afforded 3-ethynyl-2-methyl-2-buten-4-olide (4,85percent) and 2-ethynyl-3-methyl-2-buten-4-olide(5,80percent), respectively.Their formation presumably entails an ene reaction between the methylacetylenic and the acetylenic functions of the diyne esters 1 and 2 to give the two methyliden-vinyliden-butanolides 10 and 11 as intermediates, followed by a <1,5>-H shift to 4 and 5.At 400-450 deg, the gas phase flow thermolysis of 1 and 2 led to the dimers 16 (77percent) and 17 (6percent), respectively.These products resulted from the Diels - Alder dimerization of the above mentioned intermediates 10 and 11.The regioselectivity of this dimerization is determined by a 'head-to-head' approach, with the double bond conjugated to the carbonyl group acting as the dienophile in both cases.The low yield of 17 from 2 is probably due to a further Diels-Alder reaction of the dimer 17 with its precursor 11, yielding a trimer 18 (8percent isolated).This process is not possible when starting with 1, which explains the higher yield of 16.The gas phase flow thermolysis of 2-butynyl 2-butynoate (3) at 550 deg afforded a mixture of four isomeric products, namely the two monocyclic ethynyl-butenolides 6 and 7 and the two bicyclic vinyl-butenolides 8 and 9.The formations of 6-9 are also rationalizable by initial ene-reactions, in this case two alternative ones, each involving one of the two CH3 groups of 3.This leads to two alkylidene-vinylidene-butenolides, namely 12 and 13.A <1,5>-H shift converts 12 into 6 and 13 into 7.A competing alternative <1,5>-H shift transforms both 12 and 13 to the triene 14, which electrocyclizes to the cyclohexadiene 15.The latter undergoes two alternative <1,5>-H shifts to yield 8 and 9.