Photooxygenation allylic alcohols

This problem was solved by Adam and coworkers in 1994-1998. They presented a high-yielding and diastereoselective method for the preparation of epoxydiols starting from enantiomerically pure allyhc alcohols 39 (Scheme 69). Photooxygenation of the latter produces unsaturated a-hydroxyhydroperoxides 146 via Schenck ene reaction. In this reaction the (Z)-allylic alcohols afford the (5, 5 )-hydroperoxy alcohols 146 as the main diastereomer in a high threo selectivity (dr >92 8) as racemic mixmre. The ( )-allylic alcohols react totally unselectively threolerythro 1/1). Subsequent enzymatic kinetic resolution of rac-146 threolerythro mixture) with horseradish peroxidase (HRP) led to optically active hydroperoxy alcohols S,S) and (//,5 )-146 ee >99%) and the... 

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... 

Selective formation of allylic hydroperoxides via singlet oxygen ene reaction 837 TABLE 2. Syn/anti regioselectivity in the photooxygenation of allylic alcohols 10-2s and 10-Z... 

SCHEME 33. Threo/erythro diastereoselectivity in the photooxygenation of allylic alcohols 115-117... 

In the epoxidation of the allylic alcohol shown in Eq. 6A.3b, the epoxy alcohol is obtained in 96% yield and with a 14 1 ratio of enantiofacial selectivity [40b], An interesting alternate route to the epoxide of entry 12 (Table 6A.3) has been described in which 2-r-butylpropene is first converted to an allylic hydroperoxide via photooxygenation and then, in the presence of Ti-tartrate catalyst, undergoes asymmetric epoxidation (79% yield, 72% ee) [38b]. The intermediate hydroperoxide serves as the source of oxygen for the epoxidation step. 

A family of new 1,2,4-trioxanes by photooxygenation of allylic alcohols in sensitizer-doped polymers and secondary reactions. Synthesis, 14, 2433-2444. 

Epoxy alcohols. A few years ago Mihelich1 was granted a patent for preparation of epoxy alcohols by photooxygenation of alkenes in the presence of titanium or vanadium catalysts. Adam et al.2 have investigated this reaction in detail and find that Ti(IV) isopropoxide is the catalyst of choice for epoxidation of di-, tri-, and tetrasubstituted alkenes, acyclic and cyclic, to provide epoxy alcohols. When applied to allylic alcohols, the reaction can be diastereo- and enantioselective. The reaction actually proceeds in two steps an ene reaction to provide an allylic hydroperoxide followed by intramolecular transfer of oxygen catalyzed by Ti(0-i-Pr)4. The latter step is a form of Sharpless epoxidation and can be highly stereoselective. 

Several other allylic alcohols with primary C-2 substituents have been epoxidized with good results (Table 3, entries 7-10 and 14). Epoxy alcohols have been obtained with 93-96% ee and when the catalytic version of the reaction is used, as in Table 3, entry 10, the yield is excellent. When the C-2 substituent is more highly branched, as in entries 11-13, there may be some interference to high enantiofacial selectivity by the bulky group, since the ee in two cases (entries 11 and 12) is 86%. Another example which supports this possibility of steric interference to selective epoxidation is summarized in equation (3). In this case, the optically active allylic alcohol (12) was subjected to epoxidation with bo antipodes of the titanium tartrate catalyst. With (+)-DIPT enantiofacial selectivity was 96 4 ( matched pair ), but with (-)-DIPT selectivity fell to only 1 3 ( mismatched pair ), a further indication that a secondary C-2 substituent can perturb the fit of the substrate to the active catalyst species. In the epoxidation of the allylic alcohol shown in elation (4), the epoxy alcohol is obtained in 96% yield and with a 14 1 ratio of enantiofacial selectivity. An interesting alternate route to the epoxide of entry 12 (Table 3) has been described, in which 2-r-butylpropene is first converted to an allylic hydroperoxide via photooxygenation and then, in the presence of the titanium tartrate catalyst, undergoes asymmetric epoxidation (79%... 

The photooxygenation of (12.E)-abienol has been described in connection with the chemistry of tobacco45. Rather unusually, the [4 + 2] product is only found as a minor component (< 5%), with the main products being allylic alcohols, as detected by GO and GC/MS. The (12 / )-l, 3-dienol, formed via hydrogen abstraction from the methyl group, is the major stereo- and regioisomer. 

Photooxygenation of vinylsilanes affords, after reduction, /3-silyl allylic alcohols regioselective-ly in good yields i4a c. Moreover, because of the steric demand of the silyl group, the titanium-catalyzed oxyfunctionalization described leads in high yield to (R, S )-epoxy alcohols80. 

Reactions with steroidal monoolefms Photooxygenation of A -3-methyl-5a-cholestene (1) with hcmatoporphyrin as sensitizer, followed by reduction of the initially formed hydroperoxides with methanolic sodium iodide, gives the two allylic alcohols (2) and (3). Similar oxidation of the isomeric A -2-methyl-5a -cholestene yields the... 

Recently, a study on the synthesis of novel natural secondary allylic alcohol derivatives was carried out by Helesbeux et al. [64]. The photooxygenation-reduction sequence was apphed in the prenylated xanthone series also... 

Dussault and Woller have also reported on the enantioselective photooxygenation of allylic alcohols (e.g., 112) with the aim of synthesizing 1,4-dioxygen-ated peroxides (113) [154]. The photooxygenation illustrated in Scheme 26 gave 113 in 58% yield, with good stereoselectivity. 

The 1,2,4-trioxane ring system is the key structural subunit in a number of substances with promising anti-malarial activity, and has been constructed by a number of groups using a photooxygenation approach. Thus the readily available allylic alcohol geraniol reacts with O2 to give a mixture of mono- and... 

Vinylsilanes — silyl epoxy alcohols. Photooxygenation of vinylsilancs in the presence of Ti(0-(-Pr)4 affords silyl epoxy alcohols with high regio- and diastcreosclcctivity. The conversion involves as the first step an enc reaction with O2 to provide a /3-silyl allylic alcohol, which then undergoes epoxidation. 

Allylic alcohols. Access (o photooxygenation with reductis e wori Alkoxydioxines. Dieno) ethers... 

An initial plan to transform the ketone 77 to the a-methylene ketone via photooxygenation of the ketal was unsuccessful, the intermediate allylic alcohol... 

Stereoselective photooxygenation of oxygen with allylic alcohols can show hig tivity. Thus the allylic alcohol (E)-l reac... 

Stereoselective photooxygenation of allylic alcohols. The ene reaction of singlet oxygen with allylic alcohols can show high regioselectivity and also high diastereoselec-tivity. Thus the allylic alcohol (E)-l reacts to form 2 and 3 in the ratio 96 4, but 2 is... 

Adam, W. and Klug, R, P-Stannyl allylic alcohols through photooxygenation (Schenck reaction) of vinylstannanes and reduction of the resulting aUyhc hydroperoxides synthesis and selected transformations,/. Org. Chem., 59, 2695, 1994. 

---