Vinylcyclopropylacyl and polyeneacyl radicals. Intramolecular ketene alkyl radical additions in ring synthesis

Abstract

Treatment of a variety of substituted vinylcyclopropyl selenyl esters, e.g. 11, with Bu(3)SnH-AIBN in refluxing benzene leads to the corresponding acyl radical intermediates, which undergo rearrangement and intramolecular cyclisations via their ketene alkyl radical equivalents producing cyclohexenones in 50-60% yield. By contrast, treatment of conjugated triene selenyl esters, e.g. 32, with Bu(3)SnH-AIBN produces substituted 2-cyclopentenones via intramolecular cyclisations of their ketene alkyl radical intermediates. Under the same radical-initiating conditions the selenyl esters derived from o-vinylbenzoic acid and o-vinylcinnamic acid undergo intramolecular cyclisations producing 1-indanone and 5,6-dihydrobenzocyclohepten-7-one respectively in 60-70% yields. A tandem radical cyclisation from the alpha,beta,gamma,delta-diene selenyl ester 31 provides an expeditious synthesis of the diquinane 35 in 69% yield.

Preparation of Amino-Substituted Indenes and 1,4-Dihydronaphthalenes Using a One-Pot Multireaction Approach: Total Synthesis of Oxybenzo[c]phenanthridine Alkaloids

“…Purification by filtration through a pad of silica (elution with 20% diethyl ether in petroleum ether) gave ethyl (2 E )-3-(2′-vinylphenyl)­prop-2-enoate ( 7a ) (0.331 g, 98%) as a yellow oil. Spectroscopic data were in accordance with literature values …”

Ketene Chemistry after 100 Years: Ready for a New Century

“…Ketenyl intermediates with radical character have been exploited in synthetic applications, as in the reaction of the selenyl ester 114 with n Bu 3 SnH to give the cyclopropylacyl radical 115 in hot benzene, which was proposed to open to the ketenyl radical 116 , which then forms the aldehyde 117 by ketene reduction, together with the dimer 118a , while in MeOH the ester 118b was the only observed product [Equations (46) and (47)] 24a. In a further experiment, the selenyl ester 119 gave the ketone 123 by a pathway proposed to involve the radical intermediates 120 – 122 [Equations (48) and (49)] 24b. An analogous reaction of an allenyl derivative gave a cyclooctadienone 24c…”

Some intriguing mechanistic aspects of the radical chemistry of xanthates

“…More unexpected is the absence of products arising from the ring opening of the cyclopropane. In earlier work, Pattenden and co‐workers had observed products resulting from ketene formation by ring opening of the cyclopropylacyl radical, but the ring‐opened radical was stabilized by a carbonyl group in their structure . The occurrence of a reversible ring opening in our case could be ruled out by the experiment displayed in Scheme , where the cyclopropylacyl radical 62 derived from xanthate 61 could be captured to give adduct 63 without alteration of the stereochemistry .…”