Synthetic applications allylic alcohols

The Sharpless-Katsuki asymmetric epoxidation (AE) procedure for the enantiose-lective formation of epoxides from allylic alcohols is a milestone in asymmetric catalysis [9]. This classical asymmetric transformation uses TBHP as the terminal oxidant, and the reaction has been widely used in various synthetic applications. There are several excellent reviews covering the scope and utility of the AE reaction... 

In addition to the synthetic applications related to the stereoselective or stereospecific syntheses of various systems, especially natural products, described in the previous subsection, a number of general synthetic uses of the reversible [2,3]-sigmatropic rearrangement of allylic sulfoxides are presented below. Several investigators110-113 have employed the allylic sulfenate-to-sulfoxide equilibrium in combination with the syn elimination of the latter as a method for the synthesis of conjugated dienes. For example, Reich and coworkers110,111 have reported a detailed study on the conversion of allylic alcohols to 1,3-dienes by sequential sulfenate sulfoxide rearrangement and syn elimination of the sulfoxide. This method of mild and efficient 1,4-dehydration of allylic alcohols has also been shown to proceed with overall cis stereochemistry in cyclic systems, as illustrated by equation 25. The reaction of trans-46 proceeds almost instantaneously at room temperature, while that of the cis-alcohol is much slower. This method has been subsequently applied for the synthesis of several natural products, such as the stereoselective transformation of the allylic alcohol 48 into the sex pheromone of the Red Bollworm Moth (49)112 and the conversion of isocodeine (50) into 6-demethoxythebaine (51)113. 

The method is not restricted to secondary aryl alcohols and very good results were also obtained for secondary diols [39], a- and S-hydroxyalkylphosphonates [40], 2-hydroxyalkyl sulfones [41], allylic alcohols [42], S-halo alcohols [43], aromatic chlorohydrins [44], functionalized y-hydroxy amides [45], 1,2-diarylethanols [46], and primary amines [47]. Recently, the synthetic potential of this method was expanded by application of an air-stable and recyclable racemization catalyst that is applicable to alcohol DKR at room temperature [48]. The catalyst type is not limited to organometallic ruthenium compounds. Recent report indicates that the in situ racemization of amines with thiyl radicals can also be combined with enzymatic acylation of amines [49]. It is clear that, in the future, other types of catalytic racemization processes will be used together with enzymatic processes. 

The hydrogenation of allylic alcohols and a,/>-unsaluraled acids leads to products with a very high synthetic potential, and both transformations were used quite early for industrial applications. In both cases Ru complexes with axially chiral biaryl ligands (binap analogues) are the catalysts of choice. Here, we will dis-... 

In 1980, Katsuki and Sharpless described the first really efficient asymmetric epoxidation of allylic alcohols with very high enantioselectivities (ee 90-95%), employing a combination of Ti(OPr-/)4-diethyl tartrate (DET) as chiral catalyst and TBHP as oxidant Stoichiometric conditions were originally described for this system, however the addition of molecular sieves (which trap water traces) to the reaction allows the epoxidation to proceed under catalytic conditions. The stereochemical course of the reaction may be predicted by the empirical rule shown in equations 40 and 41. With (—)-DET, the oxidant approaches the allylic alcohol from the top side of the plane, whereas the bottom side is open for the (-l-)-DET based reagent, giving rise to the opposite optically active epoxide. Various aspects of this reaction including the mechanism, theoretical investigations and synthetic applications of the epoxy alcohol products have been reviewed and details may be found in the specific literature . 

Hydrolysis of Chlorinated Hydrocarbons. The production of oxygenated aliphatics by the hydrolysis of chlorinated hydrocarbons includes the synthetic glycerol process and the amyl alcohols process. Glycerol (7) is made from propylene via allyl chloride (CH2 CHCH2C1), and competes with glycerol made from fats and oils for use in dynamite and alkyd resins, as a tobacco humectant and cellophane plasticizer, in cosmetics and pharmaceuticals, and for other applications. Amyl alcohols have been made since 1926 by the alkali hydrolysis of a mixture of amyl chlorides, made by the chlorination of pentanes from natural gasoline. Production from this source far exceeds the supply from the fusel oil by-product of fermentation processes. Amyl alcohol and its derivatives are used mainly as solvents. 

Another approach in the study of the mechanism and synthetic applications of bromination of alkenes and alkynes involves the use of crystalline bromine-amine complexes such as pyridine hydrobromide perbromide (PyHBts), pyridine dibromide (PyBn), and tetrabutylammonium tribromide (BiMNBn) which show stereochemical differences and improved selectivities for addition to alkenes and alkynes compared to Bn itself.81 The improved selectivity of bromination by PyHBn forms the basis for a synthetically useful procedure for selective monoprotection of the higher alkylated double bond in dienes by bromination (Scheme 42).80 The less-alkylated double bonds in dienes can be selectively monoprotected by tetrabromination followed by monodeprotection at the higher alkylated double bond by controlled-potential electrolysis (the reduction potential of vicinal dibromides is shifted to more anodic values with increasing alkylation Scheme 42).80 The question of which diastereotopic face in chiral allylic alcohols reacts with bromine has been probed by Midland and Halterman as part of a stereoselective synthesis of bromo epoxides (Scheme 43).82... 

The hallmark of Ti-tartrate catalyzed asymmetric epoxidation is the high degree of enantiofacial selectivity seen for a wide range of allylic alcohols. It is natural to inquire into what the mechanism of this reaction might be and what structural features of the catalyst produce these desirable results. These questions have been studied extensively, and the results have been the subject of considerable discussion [6,135,136]. For the purpose of this chapter, we review the aspects of the mechanistic-structural studies that may be helpful in devising synthetic applications of this reaction. 

Sharpless kinetic resolution of y-trimethylsilyl allylic alcohols can be highly efficient in the case shown (equation 28), the epoxyalcohol and the remaining allylic alcohol were both formed in greater than 99% ee. Further synthetic applications of the product chiral... 

Porphyrin complexes, however, are prone to oxidative decomposition and therefore synthetic applications are hampered by rapid catalyst deactivation. This problem can be overcome by attaching electron-withdrawing groups to the periphery of the porphyrin system. Another problem is the poor chemoselectivity. In many cases, addition to the C=C double bond and formation of the epoxide are much faster than the corresponding hydrogen abstraction, which leads to the allylic alcohols. This is... 

Heterogeneous copper catalysts prepared with the chemisorption-hydrolysis technique are effective systems for hydrogen transfer reactions, namely carbonyl reduction, alcohol dehydrogenation and racemization, and allylic alcohol isomerization. Practical concerns argue for the use of these catalysts for synthetic purposes because of their remarkable performance in terms of selectivity and productivity, which are basic features for the application of heterogeneous catalysts to fine chemicals synthesis. Moreover, in all these reactions the use of these materials allows a simple, safe, and clean protocol. 

In their search for suitable synthetic applications of their methodology, Shibasaki and coworkers spared no efforts and carried out an 18-step synthesis of lactone 35, which represents an early intermediate of Danichefsky s synthesis of (-i-)-vernolepin (Scheme 10) [14]. First, the ester 31 is transformed via 32 into the allylic alcohol 33, which is then cyclized with good enantioselectivities to yield the enone 34 (which is initially formed as an enol by /i-H-elimination). 

Isomerizations have wide synthetic applications. For example, [Ru(H20)6]2+ catalyzes the isomerization of allylic alcohols to carbonyl compounds,31 and propy-nylic alcohols are isomerized to a,j8-unsaturated ketones 32... 

There are many synthetic applications of epoxide formation. Titanium alkoxides in the presence of diethyltartrate as a chiral ligand catalyze the epoxidation of allylic alcohols enantioselectively (Sharpless reaction). In the presence of singlet... 

The hydroxylation of allylic C-H bonds presents a serious challenge in synthetic organic chemistry, because a number of natural products contain substructures which can be approached synthetically via allylic functionalization moreover the allylic alcohol is in itself a very useful synthon, as demonstrated by the wide application of the Sharpless epoxidation2. 

In the thirty-some years since Konig and Neumann reported that allyl triethyltin adds thermally to aldehydes to yield homoallylic alcohols, extensive studies on the mechanism and synthetic applications of numerous variants of this reaction have been reported by research groups around the world [1,2], In the thermal version of these additions the EtsSn moiety functions as a weak Lewis acid to afford the adduct via a cyclic transition state (Eq. 1). 

Another synthetic application of selenoxides is the conversion of epoxides into allylic alcohols (equation 594) [168. Dehydrogenation occurs almost exclusively away from the hydroxyl group. 

---