Cyclohexenone, from cyclopentene

The presence of the catalyst can also favor multiple Diels-Alder reactions of cycloalkenones. Two typical examples are reported in Schemes 3.6 and 3.7. When (E)-l-methoxy-1,3-butadiene (14) interacted with 2-cyclohexenone in the presence of Yb(fod)3 catalyst, a multiple Diels-Alder reaction occurred [21] and afforded a 1 1.5 mixture of the two tricyclic ketones 15 and 16 (Scheme 3.6). The sequence of events leading to the products includes the elimination of methanol from the primary cycloadduct to afford a bicyclic dienone that underwent a second cycloaddition. Similarly, 4-acetoxy-2-cyclopenten-l-one (17) (Scheme 3.7) has been shown to behave as a conjunctive reagent for a one-pot multiple Diels-Alder reaction with a variety of dienes under AICI3 catalysis, providing a mild and convenient methodology to synthesize hydrofluorenones [22]. The role of the Lewis acid is crucial to facilitate the elimination of acetic acid from the cycloadducts. The results of the reaction of 17 with diene... 

The importance of the ethylene ketal described above with respect to stereocontrol of the de-Mayo reaction is emphasized by later published works of two other teams. Fetizon and co-workers obviously followed a similar concept and carried out photocycloaddition reactions with 96a in model studies (Scheme 23) (Si). As can be seen from retroaldol product 98, exclusive a attack of the cyclohexene has taken place. Thus, the relative stereochemistry of the BC ring connection is opposite to that of taxane. Totally comparable results were obtained by Berkowitz et al. in the course of cycloaddition reactions of cyclopentene, cyclohexene, or those of a cyclohexenone ketal to the camphor derivative 96b (.84). 

Both cyclic and acyclic enone systems participate in the (trimethylsilyl)cyclopentene annelation (Table 6). a-Methylene ketones react to form spiro-fiised systems and the intermediates derived from ace-tylcyclohexanone, cyclohexenone and cyclopentanone cyclize to yield S,S- and 6,3-fiised ring systems. ... 

In the case of cycloalkenes, it is found that the size of the ring is an important factor in product distribution. Photo [2 -I- 2] cycloaddition of cyclohexenone derivatives (4) to carbomethoxy cyclobutene [47], cyclopentene [48], and cyclohexene [49] (see Scheme 5) demonstrates a gradual reversal of regioselectivity from head-to-head to head-to-tail adducts as indicated in Table 2 [50]. This result of head-to-tail products is not consistent with the dipole-dipole interaction theory. Stability of biradical intermediates is suggested to explain the reversed regioselectivity. 

The alcohol adducts from Q -(trimethylsilyl)vinyllithium and aldehydes have found many uses in organic synthesis. The ketones obtained by oxidation (eq 4) are especially good Michael acceptors and have been used in a modified Robinson annulation reaction for the construction of cyclohexenones. Cyclopropana-tion leads to cyclopropylsilane adducts, which can be converted into a variety of cyclopentenes (eq 5). Halogenation followed by stannylation gives synthons (eq 6) useful for making substituted vinylsilanes through radical reactions. ... 

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