Diols reaction with boranes

In principle, any difunctional chiral compound, such as a diol, diamine, or amino alcohol, can form a cyclic boron derivative by reaction with borane, haloboronic acid or its esters, alkylboronic acids, trihaloboranes or similar compounds. Several syntheses of this type are described in chapters D.1.3.3.3. (together with applications for allylic additions to carbonyl compounds) and D. 1.1.2.1. (with applications of a-haloboronic acids). 

Although this general principle of asymmetric induction has not been demonstrated for boron enolates, the related addition reactions of allylboranes to aldehydes (eq. [115]) (131) have been examined in this context. The reaction of chiral diol 175 with either triallyl-borane or tri- -methallylborane afforded the boronic esters 176 (Ri = H, Me) in yields exceeding 95% (132a). The addition reactions of 176 to representative aldehydes are summarized in Table 40. In all cases reported, the sense of asymmetric induction from the chiral... 

Hydroboration of bi(cyclopent-l-ene) and 3,3 4)iindene with borane, thexylborane, or (—)-isopinocampheylborane afforded, after oxidation, the corresponding meso-1,4-diols as the main products. No reaction was observed with 9-BBN.44... 

A hydrosilylation approach with the silicon being delivered intramolecularly from an OH group in 106 is also regioselective.18 Catalysis by H2PtCl6 efficiently gave the heterocycle 107 and an oxidation of the C-Si bond (cf. the similar reaction on boranes above) gave one regioisomer of the diol 108. 

PROPANEDIOL or PROPANE-1,3-DIOL (504-63-2) C3H8O2 Combustible liquid (flash point 174°F/79°C cc Fire Rating 2). Violent reaction with strong oxidizers, alkalis. Incompatible with sulfuric acid, nitric acid, caustic materials, aliphatic amines, isocyanates, boranes. On small fires, use dry chemical powder (such as Purple-K-Powder), alcohol-resistant foam, or CO2 extinguishers. 

DIOXY-UNDECANE DIOL,-l,ll (24800-44-0) Combustible liquid (flash point 285°F/141°C). Violent reaction with strong oxidizers. Incompatible with aliphatic amines, boranes, alkalis, isocyanates, perchloric acid, sulfuric acid, nitric acid. Attacks some plastics, rubber, and coatings. 

A reaction has been developed for the conversion of a 3-unsaturated ketones into 3-alkyl trans-1,2-diols. Previously it had been shown that addition of MejCuLi to cyclohex-2-enone followed by acid workup gives 3-methyl-cyclohexanone in high yidd. If, after Me2CuLi addition and separation from the black copper-containing precipitate, the reaction mixture is treated with borane, followed by oxidation with alkaline HjOj, a 55% yield of two diols (108) and (109) in a ratio of 87 13 is obtained. Likewise, isophorone (110) was converted into (111) after a two-day hydroboration step in 53% yield. 

The same authors have shown that independently prepared cyclohexanone enolate on treatment with borane yields trans-cyclohexane-l,2-diol in 45% yield. 2-Methylcyclohexanone, which can give two enolates under appropriate conditions, may thus be selectively converted into either a mixture of trans-diols (112) and (113) in a ratio of 42 58 or the trans-diol (114). Enol silyl ethers may also be employed as substrates for this reaction. 

Paterson et al. [98] in their attempt used a similar disconnection for rhizopodin as described by Menche (fragments 144 and 149) (Scheme 2.151). However, unlike, Menche, they used silyl ketene acetal 16 in an asynunetric VMAR for the addition to ( )-iodoacrolein (142) to obtain dioxinone 143 in 94% ee. Methanolysis removed the aceto-nide, and the subsequent Narasaka reduction [99] provided the syn-diol 144 in 80% yield and a 10 1 selectivity for the desired isomer. The synthesis of segment 149 started with aldehyde 145, which was ultimately derived from Roche ester. Carbon chain extension was achieved through a chelation-controlled Mukaiyama aldol reaction with silyl ketene acetal 146, which installed the new chiral center with excellent stereocontrol (20 1 dr). For the installation of the third secondary alcohol, six-membered lactone 148 was obtained by treatment with K COj in methanol. Subsequent borane reduction provided stereospecifically the desired alcohol, which was then further transformed to the desired acid (149). 

An alternative method of hydroboration is to use diisopinocampheylborane (12) (Scheme 4). This reaction is particularly useful for sterically hindered alkenes. Diisopinocampheylborane (12) is prepared from borane-dimethyl sulfide and (+)-pinene.[23-24] Treatment of 4-meth-ylenecyclohexanone ethylene ketal with diisopinocampheylborane (12) gives the borane 13.[25] Further treatment with 2 equivalents of an aldehyde results in the elimination of pinene and the formation of a new dialkyl boronate, e.g. treatment of 13 with acetaldehyde gives the diethyl cyclohexylmethylboronate 14J261 The dialkyl boronates thus produced can be transesterified with pinanediol to give 15[26] or with other cyclic diols. 

A 2-methoxyethoxymethyl ether (MEMOR) is normally prepared under non-acidic conditions in methylene chloride solution or under basic conditions. The MEM ether group can be removed in excellent yield with trifluoroacetic acid (TEA) in dichloromethane (1 1). The MEM group can also be removed by treatment with zinc bromide (ZnBr2), titanium chloride (TiCU) or bromocatechol borane. When MEM-protected diols are treated with zinc bromide (ZnBr2) in ethyl acetate, 1,3-dioxane is formed and a mechanism of this reaction is given in Scheme 1.23. 

Triphenylsilyl ethers are typically prepared by the reaction of the alcohol with triphenylsilyl chloride (mp 92-94 °C) and imidazole in DMF at room temperature. The dehydrogenative silylation of alcohols can be accomplished with as little as 2 mol% of the commercial Lewis acid tris(pentaf1uorophenyl)borane and a silane such as triphenylsilane or triethylsilane [Scheme 4.98]. Primary, secondary, tertiary and phenolic hydroxyls participate whereas alkenes, alkynes, alkyl halides, nitro compounds, methyl and benzyl ethers, esters and lactones are inert under the conditions. The stability of ether functions depends on the substrate. Thus, tetrahydrofurans appear to be inert whereas epoxides undergo ring cleavage. 1,2- and 1,3-Diols can also be converted to their silylene counterparts as illustrated by the conversion 983 98.4. Hindered silanes such as tri-... 

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