Singlet oxygen diastereoselectivity

Acetylenic ethers 7 can be hydrozirconated, and subsequent iododezirconation leads to (fc)-iodo enol ethers 8 (Scheme 4.4) [18], These species undergo efficient Sonogashira couplings to give (E)-enynes, which are ultimately converted to stereodefined dienol ethers. These dienes have proven useful in studies of diastereoselective cycloaddition reactions with singlet oxygen, where R in 8 is a nonracemic auxiliary (e. g., menthyl) (Procedure 3, p. 140). 

These intrazeolite singlet oxygen ene reactions have synthetic potential because the cis effect observed in solution is suppressed in the zeolite [13]. Consequently, allylic hydroperoxides which are inaccessible by other routes may be available via this new technology. For example, photo-oxidations of aryl-substituted alkenes, 7, in sensitizer-doped NaY react to generate the allylic hydroperoxides as the major or exclusive product [17]. In contrast, in solution, the hydroperoxides are formed in only 5-20% yields, with 2-1-2 and 4-1-2 adducts dominating the reaction mixtures. In the case of 2-methyl-5-phenyl-2-hexene, 8, the regio-selectivity for 8b and 8c improved from 47% to 94% and the diastereoselectivity from 10% to 44% as the reaction is moved from solution into the zeolite [18] ... 

The synthesis of the non-natural ( )-7,14-epz-l(15),8-dolastadien-7,14-ol (rac-7yl4-epi-l09) was published by Paquette in 1986 and is highlighted by a photochemical rearrangement of the 6,6,6-tricyclic a,yS-epoxy ketone 148 into the 5,7,6-tricyclic dolastane skeleton (149) (Scheme 23) [84]. The succeeding hydroxylation of carbon atom by photo oxygenation with singlet oxygen as well as a DIBAH reduction of a keto function proceeded with an undesired substrate-induced diastereoselectivity to provide the racemic 7,14-epimer of the natural dolastane 109. 

A sequence of transformations involving diastereoselective singlet oxygen photooxygenation of TBS-protected 3,5-cycloheptadienol 525 to predominantly 5yw-endoperoxide syn-526 and some anti-526, followed by reduction into the all-ci5 triol 527 as described in Scheme 146, was used by Johnson for synthesis of enantiopure methyl 2,4-dideoxyhexa-pyranosides D-528 and L-5284°5.406... 

TABLE 6. Results of the regio- and diastereoselective ene reaction of singlet oxygen with chiral aUyUc alcohols, acetates, amines (and acyl derivatives), silyl ethers and ethers... 

SCHEME 31. Diastereoselection in the singlet oxygen ene reaction of 7-isopropylidenenorbornenes... 

Control of the diastereoselectivity in the singlet oxygen ene reaction can be achieved by the use of chiral auxiliaries. Success in this field would open up promising prospects in the preparation of optically active building blocks for asymmetric synthesis, since the oxygenation reaction could be followed by the removal of the chiral auxiliary. Unfortunately,... 

SCHEME 40. Diastereoselectivity in the chiral-auxihary-controlled singlet oxygen ene reaction of optically active oxazolidines... 

It has already been mentioned (Section III) that the study of the diastereoselection in the electrophilic addition of singlet oxygen to the n face of chiral alkenes is of primary interest for the achievement of a selective oxyfunctionalization reaction. Zeolite confinement and cation- 7r interactions might be expected to affect significantly the diastereoselectivity in the photooxygenation of chiral alkenes. 

Singlet oxygen affords a variety of regio and diastereoselective reactions with chiral allylic alcohols amines - (e.g. substrate 161, Scheme 58) and chiral cyclohexadienes - that are useful for synthetic transformations. For example, the photooxygenation of a chiral allylic alcohol was used recently as the key step in the total syntheses of plakorin 162 and ewawfio-chondrilin (Scheme 58). If the photooxidation... 

Table 7 Diastereoselectivity of the [4+2] cycloaddition with singlet oxygen as dienophile...

Cycloadditions of singlet oxygen to particular fused 1,3-cyclohexadienes can be highly diastereoselective, and therefore useful in the total syntheses of some complex natural products. For example, chemically generated 02 effectively and stereoselectively adds to the termini of the 1,3-diene system in 515 to give a single endoperoxide 516 (81%),... 

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