Intermediates unstable, diene synthesis

Acyclic dienamines are relatively unstable, but very reactive towards electron-poor dienophiles as describe in a review article (1984). A more recent example shows the preparation of sterically encumbered 1-amino-1,3-diene (125) and its, nevertheless, efficient and highly stereoselective [4 + 2] cycloaddition to dimethyl fumarate (Scheme 32). Elimination of pyrrolidine, by heating the non-iso-lated cycloadduct with acetic anhydride, afforded hexahydronaphth ene (126), a key intermediate for a synthesis of drimane-type sesquiterpenes. 

An extremely efficient synthesis of lactone [41] is provided (33,34) by asymmetric synthesis (Fig. 10). Alkylation of the anion of cyclopentadiene with methyl bromoacetate gave the unstable diene [59], Immediate asymmetric hydroboiation with (+)-di-3-pinanylborane gave, after oxidative workup, the hydroxy ester [60] in about 95% e.e. Lactonization involved conversion to mesylate [61] and saponification. The crystalline lactone [41] was readily brought to an enantiomerically pure state. This route is apparently the basis for commercial quantities of compound [41], the Corey lactone, and other prostaglandin intermediates offered by the Hungarian firm Chinoin. 

Diene synthesis with unstable intermediates—o-Quinodimethanes as inter-... 

Diene synthesis with unstable intermediates o-Quinodimethanes as intermediates... 

Analysis of triene structures in leukotrienes or the dienes in the HETEs reveals in all cases that a cis,trans dienes (partial structure) is commonly shared. The Merck Frosst group has recently described a methodology for the delivery of cis,trans dienes in the synthesis of various HETEs. Scheme 2.9 depicts the basic route whereby a-diazoketones 25 react, via a carbenoid species, with furan to produce an unstable cyclopropyl furan intermediate 26. This reaction is efficiently... 

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