Dedicated to Prof. H . F. Mark, Brooklyn, in admiration and reverence on the occasion of his 95th birthday (12.X11.89) ~~~ ~~~The effect of substituents (X = H, Me, or F at C(6), R = H or Me at C(2') of the ally1 side chain) on the photoisomerization (2 = 350 nm) of 6-allylcyclohex-2-enones 1 in MeCN is studied. Subslituents X control the overall efticiency of intramolecular [2 + 21 photocycloadduct formation (@: Me > F > H) but do not exercise an influence on the orientation of addition of the exocyclic double bond to the enone C=C bond. In contrast, replacement of the prop-2-enyl (R = H) by a 2-methylprop-2-enyl (R = Me) side chain causes a change in the tricyclo[3.3.1 .O2,']nonan-6-one 4 us. tricyclo[4.2. 1.03.8]nonan-7-one (5) product ratio from 1OO:O (R = H) to roughly 2:l (R = Me) but has almost no bearing on the relative rates of conversion of 1 to products. For C(6)-unsubstituted enones laa and 1ba (X = H), the efficiency of cyclization becomes low enough so that lumiketone rearrangement to bicyclohexanones 6 and 3-isopropylcyclopent-2-enones 9 becomes competitive. Introduction. -Applications of intramolecular enone/olefin photocycloadditions in organic synthesis have been the subject of three recent reviews [l] [2]. As stated by one of the authors [l], there is still much that is not well understood about the mechanistic aspects of this important reaction. For cyclic enones bearing a side chain with an additional C=C bond at either C(2) or C(3), the regiochemistry seems to be controlled by the number of C-atoms in the chain connecting the two C=C bonds. Similarly, Agosta and coworkers showed in their studies devoted to the regiochemical control of intramolecular photochemical reactions of carbonyl-substituted hexa-l$dienes and hepta-l,6-dienes [3-61 that in such molecules two-atom bridges usually lead to crossed adducts while bridges with three atoms generally produce straight adducts. The empirical so-called 'rule of five' stating that the regioisomer formed via a five-membered cyclic biradical is always formed preferentially has been partially overemphasized in explaining these results [7], inter alia as it does not take into consideration the formation of an exciplex between the excited enone moiety and the additional double bond prior to biradical formation. Exciplexes have been shown to be the primary intermediates in both inter- [8] [9] and intramolecular [lo] enone/olefin photocycloadditions. In a study on carbonyl-substituted hepta-1,6-dienes, Agosta and coworkers [5] discussed the possibility of initial 1,7-bonding and also mentioned that the regiochemical behaviour of substrates related to 5-allylcyclopent-2-enones and 6-allylcyclohex-2-enones had proven complicated.

Dedicated to Prof. Paul dr Muyo on the occasion of his 65th birthday (26.111.90) Cyclohex-2-enones la-lc undergo photocycloaddition to tetramethoxyethylene in benzene to afford 1 -oxaspiro[3.5]non-5-enes 3 in very good yields. In MeCN as solvent, higher relative amounts of bicyclo[4.2.0]octan-2-ones 4 are formed. Results from similar experiments with the same enones and 2,3-dimethylbut-2-ene or 1,ldimethoxyethene indicate that the driving force for oxetane formation is a sufficiently large difference in redox potentials between Ered of the enone and E,, of the alkene. A mechanism is proposed for cycloalk-2-enone + alkene photocycloadditions wherein the cyclobutane adducts arise from an exciplex intermediate and the oxetanes from a subsequently formed contact ion-pair.

The photorearrangement of cyclohex-2-enones 4 a 4 to bicyclo[3.1 .O]hexan-2-ones 5 and cyclopent-2-enones 6 ( A = 350 nm, MeCN) was investigated. Both the quantum yield (Q4 = 0.004 -0.024) and the product ratio (5/6 = 65: 35-31 : 69) vary only over a rather small range, indicating the rearrangement to be relatively insensitive to substituents on C(5) or C(6). Compounds 4b, 4c, and 4g with just one alkyl group at either C(6) or C(5) rearrange selectively to the diastereoisomer 5 with alkyl group and three-membered ring in trans-configuration, while 6-fluorocyclohex-2-enones 4d and 4f afford mixtures of diastereoisomeric bicyclohexanones. Mechanistic conclusions regarding an intermediate trimethylene biradical are presented.

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