Enol ether photocycloaddition

The enol ether double bond contained within the ds-fused dioxa-bicyclo[3.2.0]heptene photoadducts can also be oxidized, in a completely diastereoselective fashion, with mCPBA. Treatment of intermediate XXII, derived in one step from a Patemo-Buchi reaction between 3,4-dimethylfuran and benzaldehyde, with mCPBA results in the formation of intermediate XXIII. Once again, consecutive photocycloaddition and oxidation reactions furnish a highly oxygenated system that possesses five contiguous stereocenters, one of which is quaternary. Intermediate XXIII is particularly interesting because its constitution and its relative stereochemical relationships bear close homology to a portion of a natural product known as asteltoxin. 

Schreiber and his coworkers have published extensively over the past decade on the use of this photocycloaddition for the synthesis of complex molecules730 81. Schreiber was the first to recognize that the bicyclic adducts formed in these reactions could be unmasked under acidic conditions to afford threo aldol products of 1,4-dicarbonyl compounds (175 to 176) (Scheme 40). The c -bicyclic system also offers excellent stereocontrol in the addition of various electrophilic reagents (E—X) to the enol ether of these photoadducts on its convex face (175 to 177). This strategy has been exploited in the synthesis of a variety of architecturally novel natural products. 

The Pirrung synthesis is notable for its brevity and clever amalgamation of [2 + 2] photocycloaddition and Wagner-Meerwein rearrangement chemistry Enol ether 757 was reacted with the Grignard rea nt from 5-bromo-2-methyl-l-pentene, subjected to acid hydrolysis, and irradiated to generate the tricycle 738. Wittig olefination of this ketone and treatment with p-toluenesulfonic acid provided racemic isocomene. 

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

Photocycloadditions of silyl enol ethers 362 with aromatic aldehydes 363 have been used to prepare substituted oxiranes with high diastereoselectivities favoring the isomer... 

The photochemical addition of cyclic 1,3-diones such as dimedone, 1,3-cylohexandione 62, or their respective silyl enol ethers leads to the formation of two fused furanylfullerenes, (1) achiral 63 and (2) chiral 64 [244], The latter having an unusual bis-[6,5] closed structure. In the initial step of this reaction, [2 + 2] photocycloaddition across a [6,6] bond to form cyclobutanols or the corresponding TMS ethers is involved (Scheme 26). Oxidation with 02 yields in the formation of the radical 65a. Cleavage to 66a followed by cyclization gives furanyl radical 67a. H abstraction by 102 or a peroxy radical finally leads to product 63. In competition, formation of fullerene triplets by absorption of a... 

The [2 + 2]-photocycloaddition of nonconjugated alkenes was described in Chapter 5. This strategy can be used for synthesizing macrocyclic rings by using a long linker between the two alkene moieties (Scheme 9.47). Thus, bicyclic cyclobutanes can be obtained starting from enol ethers and cinnamates by means of electron-transfer sensitizers, such as DCN or DCA [80], and triplet photosensitizers such as benzo-phenone (BP) [81]. 

The regioselectivity of the Paterno-Biichi reaction with acyclic enol ethers is substantially higher than with the corresponding unsymmetrically alkyl-substituted olefins. This effect was used for the synthesis of a variety of 3-alkoxyoxetanes and a series of derivatives [55]. The diastereoisomeric cis-and tnms-l-methoxy-l-butenes were used as substrates for the investigation of the spin state influence on reactivity, regio- and stereoselectivity [56]. The use of trimethylsilyloxyethene 62 as electron rich alkene is advantageous and several 1,3-anhydroapiitol derivatives such as 63 could be synthesized via photocycloaddition with l,3-diacetoxy-2-propanone 61 (Sch. 17) [57]. 

The [2+2]photocycloaddition of 5-arylfuran-2,3-diones (496) to trimethylsilyl-oxyethylenes (495) occurs with high regio- and stereoselectivity to efficiently yield polyfunctionalised cyclobutanes (497). ° A convenient four step stereoselective synthesis of ( + )-norasteriscanolide (500) has used the [2+2]photoadduct (499) from 2-cyclopentenone and the trimethylsilyl enol ether (498) to assemble the 5/8 ring system present in many terpenoids. 2-Naphthaldehyde undergoes regio-and stereoselective [2+2]photoadditon to the cyclic ketene silyl acetals (501) to... 

The degree of asymmetric induction in the photocycloaddition reaction can be quite high with substrates containing a stereogenic center. Winkler, Scott and Williard have reported that irradiation of 1-tryptophan-derived vinylogous amide 107 led to the isolation of ketoimine 108 in 91% yield as a single diastereomer . Closure to the tetracyclic portion of the aspidosperma ring system 109 was achieved in two steps by formation of the silyl enol ether with LDA and r-butyl dimethylsilyl triflate followed by treatment with tetrabutylammonium fluoride (TBFA). Conversion of 109 to 110 with >97% optical purity was then achieved. As 110 is an intermediate in Biichi s synthesis of vindorosine, the sequence outlined in Scheme 26 represents a formal total synthesis of vindorosine. 

I, 5-dicarbonyl compounds and their interesting and varied chemistry, e.g., the formation of cyclohexenones from aldol condensation of the products. As an extension of the de Mayo reaction (the photocycloaddition of enolated /i-diketones to double bonds) enol esters, enol ethers, vinylogous esters and amides, and dioxinone have been employed as the enone components. Some intermolecular examples have already been discussed in Section 1.6.1.4.2.1.6. (cf. Table 4, entries 2 and 3) and some intramolecular systems are collected in Table 5, entries 10,... 

He found that this is also true in the [2 + 2] photocycloaddition between 1,1-disubstituted silyl enol ethers (e.g., 75a) [83a] and benzaldehyde, as in Scheme 43. The regiochemistry is largely determined on the basis of the most stable biradical, which, before closing, arranges itself so that the bulkiest substituents are trans to each other. Hence, in the cases of 1,1-disubstituted and trisubstituted silyl enol ethers, sterics is the determining factor in the stereoselectivity of the Patem6-Buchi reaction. 

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