Photolysis cyclic acetals

It has been stated that bridged bicyclic systems fail to yield any cyclic acetal upon photolysis,[Pg.382]

In this Section, hydrolysis, acetolysis, and isomerization of acetals are considered. Selective deprotection of acetals may also be achieved through halogenation, hydrogenolysis, ozonolysis, and photolysis, but these topics, are covered in an accompanying article in this Series, on the reactivity of cyclic acetals of aldoses and aldosides,3 and will not be discussed here. 

The photochemistry of cyclobutanones possessing a spiro-fused three-membered ring at the a-position has also been reported (15b). Spiro[2.3]hexan-4-one[64a], for example, undergoes photolysis in methanol solution to cyclic acetals [65a] and [66a] in addition to 3-cleavage ester [67]. The dialkylated derivative [64b], on the other hand, affords ring-expanded acetals [65b] and [66b]... 

Garcia, H., Iborra, S., Miranda, M. A., and Primo, J., Photolysis of cyclic acetals of aryl benzoyl acetates as the key step in a new synthesis of flavones, Heterocydes, 24, 2511,1986. 

The photolysis of cychc ethers with an excess of HgO-Ij reagent in benzene gives the corresponding iodoalkyl formates in one step tetrahydrofuran can be transformed into (O-iodoalkyl formate in 31% yield (Scheme 47). Aldehyde cyclic acetals lead to monoesters of ot,(D-alkanediols in one step (Scheme 48). The formation of the products can be rationalized by postulating iodoxyl radical participation, as outlined in the schemes. 

Another transient aminoxyl radical has been generated , and employed in H-abstraction reactivity determinations" . Precursor 1-hydroxybenzotriazole (HBT, Table 2) has been oxidized by cyclic voltammetry (CV) to the corresponding >N—O species, dubbed BTNO (Scheme 9). A redox potential comparable to that of the HPI —PINO oxidation, i.e. E° 1.08 V/NHE, has been obtained in 0.01 M sodium acetate buffered solution at pH 4.7, containing 4% MeCN". Oxidation of HBT by either Pb(OAc)4 in AcOH, or cerium(IV) ammonium nitrate (CAN E° 1.35 V/NHE) in MeCN, has been monitored by spectrophotometry , providing a broad UV-Vis absorption band with A-max at 474 nm and e = 1840 M cm. As in the case of PINO from HPI, the absorption spectrum of aminoxyl radical BTNO is not stable, but decays faster (half-life of 110 s at [HBT] = 0.5 mM) than that of PINO . An EPR spectrum consistent with the structure of BTNO was obtained from equimolar amounts of CAN and HBT in MeCN solution . Finally, laser flash photolysis (LFP) of an Ar-saturated MeCN solution of dicumyl peroxide and HBT at 355 nm gave rise to a species whose absorption spectrum, recorded 1.4 ms after the laser pulse, had the same absorption maximum (ca 474 nm) of the spectrum recorded by conventional spectrophotometry (Scheme 9)59- 54... 

The use of /V-chloramines, in principle, allows the facile generation of aminyl radicals upon UV photolysis in neutral media. A radical chain can be envisioned for the formation of 2-chIoromethylpyrrolidines (Scheme 7). In practice, however, there is a slow step in this sequence, step A and/or B, such that other reaction pathways, disproportionation or H-abstraction from the solvent, compete. Surzur has studied the reaction in Scheme 7 in the alcoholic solvents MeOH and /-PrOH, which serve as hydrogen atom sources, and achieved acceptable ratios of cyclic products 25 and 26 to acyclic amine 27 (70TL3107). Other /V-chloroalkenylamines gave similar results (71TL903 80TL287). /8-chloro-substituted amine products such as 25 were the sole products when the reactions were carried out in acetic acid-water mixtures these reactions involve aminium cation radicals and are discussed further in Section III,B. 

When acetylenes are irradiated in aqueous solution [27], in acetic acid [28], or in alcohols [29,30], photoaddition reactions take place to give a ketone, an enol acetate, or an enol ether, respectively. In the photohydration reaction, a hydrated proton attacks the singlet excited state of the acetylene directly [31], On the other hand, alcohols give addition products by attack on the excited states of acetylenes in a radical-like mechanism. Radical photoaddition to acetylenes occurs also with saturated hydrocarbons such as cyclohexane [29], and with cyclic ethers such as tetrahydrofiiran [32], Simple acetylenes are photoreduced on irradiation in hydrocarbon solvents for example photolysis of dec-l-yne or dec-5-yne in pentane gives the corresponding alk-ene (dec-l-ene or trans- and ds-dec-5-enes) [32]. 

Rate constants for interaction of triplet excited states of cyclic enones with alkenes were first reported by Schuster et al. > > using transient absorption spectroscopy (nanosecond flash photolysis). The rate constants were obtained from the relationship (Xx)" = ( o) + (alkene), where Xq is the limiting triplet hfetime of the enone at a given concentration in the absence of alkene. The decay of enone triplet absorption at 280 nm could be conveniently followed upon excitation of the enones (cyclopentenone [CP], 3-methylcyclohexenone [3-MCH], testosterone acetate [TA], and BNEN [4] were aU studied]) in acetonitrile and cyclohexane at 355 nm using the third harmonic of a Nd YAG laser. In aU cases, triplet decays were clearly first order. Quantum efficiencies for capture of enone triplets by alkenes (O,.) are given by fc x (alkene) using the experimentally determined values of and Xq,... 

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