Cyclohexenone rearrangements

Addition of 2-butenyl sulfone anions to 2-cyclopentenone and 2-cyclohexenone at low temperatures ( — 85 °C) gives mixtures of y-1, 4- and a-1,2-addition products. When these reactions are warmed to 1 2CC, then y-l,4-addition products predominate7,8. The lithium salts of the a-1,2-adducts rearrange to 1,4-adducts at 0°C. 

Dauben et al. have investigated the scope of the photochemical type A rearrangement/60 They conclude that the rearrangement occurs only if the fourth carbon atom of the 2-cyclohexenone ring is fully alkyl-substituted. If this requirement is not met, photodimers are the major products. This substituent requirement is necessary but not sufficient to ensure rearrangement since the presence of other groups can inhibit the reaction. 

Carbonyl insertion is preferentially observed in the photoindueed reaction of 22 to give the cyclohexenones 25 and 26 as shown in Scheme 9 [17]. The acyl complex 24 is involved as an intermediate. The eyclohexenone formation appears to be susceptible to conformational effect, as observed in the facile rearrangement of 27 to 28. 

Irradiation of 4,4-dialkyl-2-cyclohexenones affords bicyclo[3.1. OJhexanones in a so called lumiketone rearrangement (3.22) 337). The reaction proceeds with inversion of configuration on the disubstituted C-atom (3.23) 338). 

Conformationally constrained 2-cyclohexenones which cannot undergo the lumiketone rearrangement (cf. chapter 3.1.5) are efficiently reduced to cyclohexanones (4.18) 420). 

Acid catalyzed the rearrangement of the 2-alkyl-2-vinylcyclobutanones (244) to yield either cyclopentenones (245), (246) or cyclohexenones (247) via 1,2-acyl migration or 1,3-acyl migration respectively (Table 15)82). 

Ring enlargement.1 A new route to seven-membered ring systems from a cyclohexenone (1) involves a photocycloaddition of ethylene to provide the bicy-clooctanone 2. Addition of lithio-1,3-dithiane to 2 provides the adduct 3, which on reaction with HgO and HBF4 forms an unstable rearranged hydroxy aldehyde... 

Ring contraction of cyclopentenone and cyclohexenone derivatives to cyclobutanes was observed during direct irradiation of various j5,y-unsaturated cyclohexenones. By way of a Nor-rish type I cleavage in n,7i excited singlet (5t) and n,7t excited triplet (T2) states, an acyl/allyl diradical is formed which recombines in the allyl position to form the product of a 1,3-acyl migration,102 while sensitized irradiation leads mainly to oxa-di-rr-methane rearrangement (see Section 4.2.3.). 

Photochemical rearrangement by direct irradiation of /fy-unsaturated cyclohexenones resulted in a singlet 1,3-acyl shift. In this manner, irradiation of 2,2-dimethylbicyclo[4.4.0]dec-l (6)-cn-3-one in benzene gave 5-isopropylidenespiro[5.3]decan-l-one (2).57... 

A diradical is the probable intermediate in the nearly quantitative photochemical rearrangement of the tricyclic cyclohexenone 24 to the tricyclic cyclobutanone 25.81... 

If 2-methylenecyclobutanones, which themselves are not capable of this tautomerization and the following rearrangement, bear alkyl substituents with an a-hydrogen in the 3-position, still another rearrangement, a [1,3] shift, precedes the actual ring-enlargement step. This is important, because either 3-alkyl-2-methylene- or 2-alkylidenecyclobutanones or mixtures of both may be formed via [2 + 2] cycloaddition of allenes92 and can all be used as precursors for the same cyclohexenones. 

This vinylation reaction was used in a two-step synthesis of 6 in about 60% yield from cyclohexenone. This product is of interest because the derived triene (7) is converted upon heating into the 1,6-cyclododecadiene 8 by a Cope rearrangement followed by a Claisen rearrangement,4... 

N. Chida, M. Ohtsuka, K. Ogura, and S. Ogawa, Synthesis of optically active cyclohexenones from carbohydrates by catalytic Ferrier rearrangement, Bull. Chem Soc. Jpn. 64 2118 (1991). 

Skeletal rearrangements similar to the santonin-lumisantonin rearrangement have also been reported for some 2-cyclohexenones, such as A4-cholesten-3-one413 and 4,4-dimethylcyclohexenone.414... 

Irradiation of 4,4-dimethyl-2-cyclohexenone (Formula 83) in tert-butyl alcohol gives 6,6-dimethylbicyclo[3.1.0]hexan-2-one (Formula 84) in good yield with small amounts of 3-isopropyl-2-cyclopentenone (Formula 85) (46). The 3-isopropyl-2-cyclopentenone is formed by photochemical rearrangement of the bicyclic ketone (Formula 84) (46). 

A 2-cyclohexenone derivative can be transformed into the corresponding epoxy tosyl-hydrazone by sequential treatment with peracid and tosylhydrazine. The elimination of nitrogen and p-toluenesulfinate and fragmentation after rearrangement to the 3-tosylazo allylic alcohol may occur under mild conditions. Carbonyl compounds with 5,6-triple bonds are formed in high yields (J. Schreiber, 1967 M. Tanabe, 1967). If one applies this reaction to a 9,10-epoxy-1-decalone, a ten-membered 5-cyclodecyn-l-one ring is formed (D. Felix, 1971). This product is an important intermediate in the perfume industry and has been used on a large scale. For this purpose Eschenmoser developed a synthesis in which the readily removed styrene was split off instead of a sulfmic acid. Thus a l-amino-2-phenylaziridine hydrazone was used instead of a tosylhydrazone (D. Felix, 1968). ... 

Keywords 1,4-pentadiene derivatives, 2-cyclohexenone derivatives, di-7i-mcthanc rearrangement, photoirradiation... 

A very plausible mechanism for this would involve loss of hydrogen chloride from XXII to form the bicyclohexanone (XXXII) this is a cyclopropanone derivation and would certainly react at once with alkali to give XXXI. The problem is to explain how this comes about. One possibility might be a-elimination of HC1 from XXX to form a carbene but this seems unlikely such a carbene would in any case be expected to rearrange to cyclohexenone rather than to XXXII. Another possibility would be an internal displacement of chloride ion from the conjugate base of XXX, as indicated in XXXIII this, however, is sterically improbable since in the grouping —CH2—CH=-CO—CHC1— the... 

Rearrangement ofl,2-dialkyl- y3-cyclohexanediones.1 Reaction of these substrates with the anion (LDA) of this phosphonate results in rearrangement to 3-alkyl-2-cyclohexenones in 60-93% yield. Addition of ClSi(CH3)3 improves the yield. The... 

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