Allenes photochemical cycloaddition

Enamine carbaldehyde 76 (Scheme 20) by sequential photochemical cycloaddition and iminium ion-propargylsilane cyclimtion furnishes allenes 77a-c in good yield and with high diastereoselectivity (92X2081). (Gas chromatographic fR values and thin-layer chromatographic Rp values have been reported.) Radical cyclization of ca-iodoalkyl isoquinolone 78a under... 

Thermal and photochemical cycloaddition reactions of 27r-electron species represent an important synthetic approach to four-membered rings. The reactions summarized in this section include 2 + 2 cycloaddition reactions of thioketones, thioketenes, isothiocyanates, sulfenes and iminosulfenes with alkenes, allenes, ketenes, ketenimines and alkynes. 

A contrast can be seen between photochemical cycloaddition of ketenes, which show reverse regioselectivity and that of allenes (cf. Table 5, entries 12 and 13128,129). 

Maradyn, D.J., Sydnes, L.K., and Weedon, A.C., Origin of the regiochemistry in the photochemical cycloaddition reaction of 2-cyclopentenone with allene — trapping of triplet 1,4-biradical intermediates with hydrogen selenide. Tetrahedron Lett., 34, 2413-2416,1993. 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

The first step is a photochemical [2+2] cycloaddition. You could not have predicted the regio-chemistry, but it is typical of the cycloaddition of allenes with unsaturated ketones. 

This chapter reviews thermal [2 + 2] cycloadditions of alkenes to form cyclobutanes, extending the coverage provided by earlier summaries of the literature.It docs not deal with [2 + 2] cycloadditions that are achieved photochemically, or that utilize allenes, arynes or ketenes, or that are deliberately catalyzed, or that result in heterocyclic four-membered rings, even though these other varieties of [2 + 2] cycloaddition reactions are important and of substantial interest. 

High regioselectivity is found in the photochemical addition of allene to the enones (98). With enone (98a) cycloaddition affords (99) while (98b) yields the adduct (100). This compound was used for subsequent reactions in syntheses of taxane-like molecules. 

Intramolecular Additions - The predominant photochemical reaction of the allene derivatives (65) is (2 + 2)-cycloaddition yielding the housanes (66). The reaction occurs in a variety of solvents (e.g. hexane, acetonitrile or acetone) and the triplet excited state of (65) is implicated. In hexane and... 

Numerous examples for the synthesis of cyclopropanes by [2-f 1] cycloaddition of photochemically generated acylcarbenes to alkenes, allenes, 1,3-dienes, and cycloheptatriene are given in Houben-Weyl Vol. 4/5b, pp 1158-1257 and in Vol. E19b, pp 1099-1107 and pp 1300-1303. 

A study of the influence of silica gel surfaces on the photochemical addition of allene to a series of cyclic enones has been carried out. Using this technique cycloaddition occurs to the more hindered face of the enone. A typical example of allene addition, where addition takes place to the less hindered face, is reported by Piers et al. in the photoaddition to the enone (65) which affords the four isomeric products (66) in ratios of 40 51 6 3. The two major products (66a) and (66b) were used as starting materials for total syntheses of naturally occurring compounds of the stemodane type. ... 

Additions to Cyclopentenones and Related Systems. (2 + 2)-Cycloadditions are reported following the irradiation of mixtures of alkyl and aryl 2-thioxo-3/f-benzoxazole-3-carboxylates with alkenes. Cycloaddition also occurs to the CS double bond. The photochemical additions of arylalkenes to 3-phenylcyclo-pentenone and 3-phenyl cyclohexenone have been studied. The regio- and stereochemistry observed in the additions has been rationalized in terms of the stability of the intermediate biradicals. Photocycloaddition of allene to the cyclopentenone derivative (6) in methylene chloride solution at — 78°C affords... 

Nitrogen-containing heterocyclic enone systems also reacted with allene to give (2 + 2)-cycloadducts, as was shown in alkaloid synthesis.178 As the keystep in the synthesis of an annotinine derivative, allene was added to 157 and the adduct 158 was obtained in quantitative yield. Various uracils have been modified by photochemical (2 + 2)-cycloaddition with olefins, e.g., vinylene carbonate,17,18° vinyl ethers, vinyl acetates, and (cetene acetals yielded 159.181 Very recently the photochemical addition of cyanoethylenes to 2-pyridones has been observed to yield mixtures of tetrahydroazocin-2-ones (160) and (2 + 2)-cycloadducts (161).182... 

Intramolecular cyclization of (76) in the presence of NaH gave exo (77) and endo (78) products (Equation (9)) <90SL584). Allene-based electrophile-mediated cyclization afforded iV-tosyl iodohexa-hydroazocines <94JCS(Pl)3549>. Photochemical intramolecular cycloaddition of phthalimide in acetone afforded the ring-opened product <82TL498l>. An unstable enaminone azocine derivative (81) was formed after the desulfurization of the bicyclic aminal (80), which in turn was synthesized from... 

In 1984, Schreiber published a synthesis of the pheromone in which the majority of steps involve pericyclic reactions. Make sure you understand each one as it appears—re-read the appropriate part of Chapter 34 or this chapter if you have any problems. The first step is a photochemical [2 + 2] cycloaddition. You could not have predicted the regiochemistry, but it is typical of the cycloaddition of allenes with unsaturated ketones. 

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