Regiochemical Trends in Intramolecular [2 + 2] Photocycloadditions of 6‐(prop‐2‐enyl)cyclohex‐2‐enones and 5‐(prop‐2‐enyl)cyclopent‐2‐enones

Abstract

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.

Photochemical Synthesis and Some Reactions of 7‐Oxa‐and 7‐Thiatricyclo[3.2.1.03,6]octan‐2‐ones

“…The enone is then purified by bulb-to-bulb distillation. H-NMR(CDC1,):8.45,5.72(2d,J=2.4,2H);5.62(m, lH);5.19(m,2H);2.90,2.75(dd,J= 7 129 (d); 121 (t); 107 (d); 97 (s); 38 (t); 25 (4). MS: 124 (100, M'), 95.…”

“…2-Acetyl-2-(2-methylprop-2-enyl)furan-3(2H)-one (lc). From 4c, 4 178 (d); 138 (s); 116 (1); 106 (d); 97 (3); 42 (t); 25 ( 4 ) ; 24 (4). MS: 180 (3, M'), 42.…”

“…-Intramolecular enone/olefin photocycloadditions have gained increasing importance in organic syntheses [l-31. In the course of our studies on the photochemistry of cyclic C C ,~ -unsaturated carbonyl compounds, we contributed to this topic with results on regiochemical trends in such reactions for 5-(prop-2-enyl)cyclopent-2-enones [4]. We also emphasized on the synthesis of several corresponding heteropentacyclic enones having a heteroatom instead of C(4), e.g.…”

“…This is reflected in the behavior of compounds 1 and 2 in intramolecular photocycloadditions, where always only one orientation of addition of the exocyclic double bond to the enone C=C bond (formation of 10 and 11) was observed. In the irradiation of 5-(prop-2-enyl)-or 5-(2-methylprop-2-enyl)cyclopent-2enones, both isomeric tricyclo[3.2.1 .03~6]octan-2-ones and -7-ones were commonly formed [4] [12], the former usually being the major reaction product.…”

ChemInform Abstract: Synthesis and Photochemistry of 5,5‐Dimethyl‐1H‐pyrrol‐2(5H)‐one and of Some N‐Substituted Derivatives.

“…Introduction. ~ In the course of our investigations on the photochemistry of cyclic a$-unsaturated carbonyl compounds [l] [2], we had emphasized on the synthesis of several heteropentacyclic enones such as 1 and 2 with a geminal dimethyl group adjacent to the heteroatom to prevent tautomerization to a 2-or 3-hydroxy-substituted heterocycle. We had shown that both 3(2H)-and 2(5H)-furanones l a and 2a and 3(2H)-thiophenone l b exhibit photochemistry typical for cyclopent-2-enone itself and that 2(5H)-thiophenones, e.g.…”