Stereocontrolled Photooxygenations–A Valuable Synthetic Tool§

Abstract

The stereochemical course of the singlet-oxygen ene reaction with acyclic olefins may be controlled if in the substrate conformational fixation (1,3-allylic strain) an allylic substituent for interaction with the attacking oxygen enophile aligns. Various substrates were chosen to elucidate the features of the olefin that are necessary to control the sense (threo versus erythro) and the extent of the n-facial preference of the singlet-oxygen attack. Depending on the electronic properties of the double bond and the nature of the allylic substituent, threo or erythro selectivity may be imposed through hydrogen bonding, electrostatic and steric effects and stereoelectronic alignment. Such directing properties, especially that of the hydroxy group, were also confirmed in the other reaction modes of singlet oxygen, namely the [4+2] cycloaddition to chiral naphthylenic alcohols and the [2+2] cycloaddition to an adamantylidene-substituted allylic alcohol. The syntheses of the natural products Merucathin and Isodihydromahubanolide B are two examples in which such stereocontrolled photooxygenations have been used as key steps to build up the required chirality diastereoselectively.

Synthesis of 4,6-Dideoxyfuranoses through the Regioselective and Diastereoselective Oxyfunctionalization of a Dimethylphenylsilyl-Substituted Chiral Homoallylic Alcohol

“…The silyl group at the double bond in the homoallylic alcohol 1 directs the H abstraction regioselectively to the gem allylic position in the singlet-oxygen ene reaction ( gem effect). , Only traces (<5%) of the regioisomeric hydroperoxide 2 ‘ were produced, which immediately decomposed to the enone 3 and dimethylphenylsilanol. The diastereomeric hydroperoxy alcohols 2a ( like ) and 2b ( unlike ) were obtained as a 50:50 mixture, the lack of diastereoselectivity in this photooxygenation is due to the absence of 1,3-allylic strain. 9b, …”

A Comparison of Intrazeolite and Solution Singlet Oxygen Ene Reactions of Allylic Alcohols

“…2). Allylic alcohol, 4-methyl-3-buten-2-01, 6, was a key substrate in the elegant work of Adam and coworkers (9,(15)(16)(17)(18)(19)(20) that was used to establish the importance of hydrogen bonding as a directing element in singlet oxygen ene reactions. Three rotomers of the S enantiomer of 6 are shown in Fig.…”

Supramolecular Control of Singlet Oxygen Generation

“…The increased oxidative power and electronic configuration of 1 O 2 has given rise to its use as a synthetic tool ( Figure 2 ) to access numerous organic intermediates that are not easily achieved through classical organic transformations [ 23 , 24 , 25 ]. Reactions such as singlet-ene [ 26 , 27 ], cycloaddition (4 + 2 and 2 + 2) [ 28 ], and heteroatom oxygenations [ 29 , 30 ] have been useful in fine chemical and large-scale synthesis of numerous natural products and drugs (include pictures of reactions). A recently fast-growing interdisciplinary area of research involving singlet oxygenation is designing functional systems such as self-cleaning [ 31 ] and disinfecting materials [ 22 , 32 ], and lasers for solar energy conversion [ 33 ].…”