Homoallylic alcohols asymmetric synthesis

Hodgson et al. showed that a series of bis- and tris-homoallylic terminal epoxides underwent intramolecular cydopropanation to give a range of bicydic alcohols. A short asymmetric synthesis of sabina ketone based on this chemistry was demonstrated (Scheme 5.20). A practical advantage with this process is that the volatile epoxides can be replaced with readily available chlorohydrins, an extra... 

B. Potassium allyl- and crotyltrifluoroborates undergo addition to aldehydes in biphasic media as well as water to provide homoallylic alcohol in high yields (>94%) and excellent diastereoselectivity (dr >98 2). The presence of a phase-transfer catalyst (e.g., B114NI) significantly accelerates the rate of reaction, whereas adding fluoride ion retards the reaction (Eq. 8.70).165 The method was applied to the asymmetric total synthesis of the antiobesity agent tetrahydrolipstatin (orlistat).166... 

During the past 2 years several research groups have published research that either uses or expands upon Crowe s acyclic cross-metathesis chemistry. The first reported application of this chemistry was in the synthesis of frans-disubstitut-ed homoallylic alcohols [30]. Cross-metathesis of styrenes with homoallylic silyl ethers 15, prepared via asymmetric allylboration and subsequent alcohol protection, gave the desired trans cross-metathesis products in moderate to good yields (Eq. 15). 

Efforts have been made to apply r 3-allyltitanium chemistry to the asymmetric synthesis of homoallylic alcohols and carboxylic acids. The synthesis and reactions of chiral r 3 -allyl-titanocenes with planar chirality, or containing Cp ligands with chiral substituents, have been reported [6c,15,30—32]. The enantiofacial selectivity in the allyltitanation reactions has been examined for the complexes 12 [15], 13 [30], 14 [31], 15, 16, and 17 [32] depicted in Figure 13.2. 

The enantioselective addition of an allylsilane to an aldehyde catalyzed by chiral acyloxyborane (CAB) 13 is an excellent method for obtaining optically active homoallyl alcohols.Itsuno and Kumagai reported that the synthesis of a new optically active polymer with chirality on the mainchain is possible by applying this reaction to the asymmetric polymerization of bis(allylsilane) and dialdehyde (Scheme 12.11). ... 

Bartlett PA, Johnson WS, Elliott JD (1983) Asymmetric synthesis via acetal templates. 3. On the stereochemistry observed in the cyclization of chiral acetals of polyolefinic aldehydes formation of optically active homoallylic alcohols. J Am Chem Soc 105 2088-2089... 

N atom of which carries a chiral substituent, react with aldehydes with high se-lectivities with formation of homoallyl alcohols of type 4, which were used in the asymmetric synthesis of complex molecules [12]. 

Scheme 1.3.6 Asymmetric synthesis of sulfonimidoyl-substituted homoallyl alcohols.

Chiral alkenyl and cycloalkenyl oxiranes are valuable intermediates in organic synthesis [38]. Their asymmetric synthesis has been accomplished by several methods, including the epoxidation of allyl alcohols in combination with an oxidation and olefination [39a], the epoxidation of dienes [39b,c], the chloroallylation of aldehydes in combination with a 1,2-elimination [39f-h], and the reaction of S-ylides with aldehydes [39i]. Although these methods are efficient for the synthesis of alkenyl oxiranes, they are not well suited for cycloalkenyl oxiranes of the 56 type (Scheme 1.3.21). Therefore we had developed an interest in the asymmetric synthesis of the cycloalkenyl oxiranes 56 from the sulfonimidoyl-substituted homoallyl alcohols 7. It was speculated that the allylic sulfoximine group of 7 could be stereoselectively replaced by a Cl atom with formation of corresponding chlorohydrins 55 which upon base treatment should give the cycloalkenyl oxiranes 56. The feasibility of a Cl substitution of the sulfoximine group had been shown previously in the case of S-alkyl sulfoximines [40]. 

The ability of allyltin halides to extend their coordination sphere allowed the preparation of chiral hypervalent complexes with diamine ligands, which have been efficient in the asymmetric synthesis of homoallylic alcohols with up to 82% ee100. Similarly, a chiral hypervalent allyltin was prepared from a low valent tin (II) catecholate, chiral dialkyl tartrate and ally lie halide101. The allylation of aldehydes and activated ketones proceeded with high enantiomeric excess. Allyltins prepared from Lappert s stannylene and allylic halides were shown to be efficient as well, although the Lewis acid character of the tin atom is much less marked in that case102,103. 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

The ruthenium-catalyzed olefin cross-metathesis to the preparation of functionalized allyl boronates has resulted in a one-pot three-component coupling procedure for the synthesis of functionalized homoallylic alcohols.617,618 The utility of the protocol was demonstrated in asymmetric allylboration using a tartrate ester (Equation (152)).617... 

Asymmetric allylboration has also been applied to y-methoxyallyl derivatives. Isomerically pure (Z)-y-methoxyallyldiisopinocampheylborane (rf31), prepared from Ipc2lSOMe and the lithium anion of allyl methyl ether, reacts with various aldehydes to afford the yyn - j-m e (boxy homoallylic alcohol (32a) in a highly regio- and stereoselective manner17 (Scheme 3.In). This one-pot synthesis of enantiomerically pure 1,2-diol derivatives went as smoothly as the asymmetric Brown crotylation, affording products with uniformly high diastereoselectivity. 

Corey s asymmetric allylation methodology was utilized in the total synthesis of amphidinolide T3 (95), a marine natural product that exhibits significant antitumor properties37 (Scheme 3.1gg). The asymmetric allylation of the aldehyde 96 was carried out successfully with chiral allylborane reagent generated in situ from allyltributylstannane and (R,R)-82 to furnish the homoallylic alcohol desired (97) in 85% yield with excellent diastereoselectivity. Subsequent conversion of the alcohol to the tosylate ester followed by treatment with potassium hydroxide resulted in formation of the trisubstituted tetrahydrofuran 98. 

Asymmetric hydroboration 2171 of prochiral alkenes with monoisopinocampheyl-borane in the molar ratio of 1 1, followed by a second hydroboration of non-prochiral alkenes with the intermediate dialkylboranes, provides the chiral mixed trialkylbo-ranes. Treatment of these trialkylboranes with acetaldehyde results in the selective, facile elimination of the 3-pinanyl group, providing the corresponding chiral borinic acid esters with high enantiomeric purities. The reaction of these intermediates with base and dichloromethyl methyl ether provides the chiral ketones (Eq. 130)2l8>. A simple synthesis of secondary homoallylic alcohols with excellent enantiomeric purities via B-allyldiisopinocampheylborane has been also reported 219),... 

Asymmetric epoxidation of homoallylic alcohols. Sharpless asymmetric epoxidation of primary homoallylic alcohols with l-( + )-diethyl tartrate proceeds with only moderate enantiomeric selectivity (23-55% ee) and opposite to that observed with allylic alcohols. Unfortunately, operation at low temperatures to improve the enantiomeric excess also retards the rate drastically. Even so, this epoxidation provides a useful synthesis of (-l-)--y-amino-P(R)-hydroxybutyric acid (1). 

Homoallylic alcohols can be asymmetrically epoxidized using a chiral vanadium catalyst equipped with the hydroxaraic acid ligand 45, as exemplified in Yamamoto s concise synthesis... 

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