From α,β‐Unsaturated Fischer Carbene Complexes to Highly Substituted 3‐Ethoxycyclopentadienes, Masked Cyclopentenones

Abstract

AbstractThe β‐amino‐substituted α,β‐unsaturated Fischer carbene complexes 3 are readily available by a four step one‐pot procedure from terminal alkynes, chromium hexacarbonyl and secondary amines (24 examples with yields of 68−99% and 7 examples with yields of 26−63%). The formal [3+2] cycloadditions of complexes 3 with different alkynes including diynes and enynes performed in donor solvents such as pyridine or acetonitrile afforded highly substituted 5‐(dialkylamino)‐3‐ethoxycyclopentadienes 7, generally in medium to excellent yields (25 examples with yields of 60−95% and 7 examples with yields of 18−53%). The steric and electronic effects of the substituents on the carbene complexes and the incorporated alkynes on the regio‐ and stereoselectivity of the ring‐forming reaction have been elaborated. An interesting 1,5‐ or, more likely, 1,2‐migration of the dimethylamino group was observed for 5‐(dimethylamino)‐3‐ethoxycyclopentadienes with trimethylsilyl and iPr substituents at C‐5. Attempted asymmetric syntheses of cyclopentadienes 7 from complexes 3 with chiral amino groups or substituents were only moderately successful. At a center of chirality in the secondary amino group, complexes of type 3 gave compounds 7 with diastereomeric excesses of, at best, 59% in yields of 54%, and with a stereogenic center in the substituent R1 attached to the vinyl group of 3, diastereomeric excesses as high as 94% could be achieved, but with poor chemical yields (21%). In general, cyclopentenones 21 could be easily obtained from the cyclopentadienes 7 under acidic conditions in very good yields (4 examples with yields of 81−98%, 1 example with an overall yield of 50% from complex 3). Intramolecular aldol reactions of dicarbonyl compounds generated by hydrolysis of cyclopentadienes 7 with acetal‐protected aldehyde or ketone carbonyl groups in either the 5‐substituent R1 or the N‐substituent R2 led to the bicyclic compounds 22 and 23. The dimethylamino group in cyclopentenones 21 could be either eliminated or transformed into other functional groups via the quaternary ammonium salts 24. The elimination product, cyclopentadienone 27 can undergo dimerization either by a formal [4+2] or [2+2] cycloaddition. Cyclopentenone 21naaa with a bromovinyl‐terminated side chain undergoes an intramolecular Heck reaction to form 5‐methyl‐4,6‐dimethylenebicyclo[3.3.0]oct‐1‐en‐3‐one (32) (37% yield). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)