1-hexene ether synthesis

An interesting comparison of catalytic activities of synthetic beidellite and montmorillonite and PILCS prepared from them was recently reported [69]. The reactions examined included secondary amine formation from cyclohexylamine, ester production from 1-hexene and acetic acid and ether synthesis from pentanol. In all cases, the reactivity of the PILCS was much lower than that of the unpillared clays, whereas the montmorillonite was generally more effective than the beidellite. These results are not surprising in that all of the reactions are Br<(Jnsted-acid-catalyzed and the observed decreased activity of the PILCS may derive from a greater loss of protons to the layers by the pillared clays. This is also in keeping with the findings of Occelli and Tindwa [5] that the PILCS are mainly Lewis acids after heat treatment. 

The synthesis of the trisubstituted cyclohexane sector 160 commences with the preparation of optically active (/ )-2-cyclohexen-l-ol (199) (see Scheme 49). To accomplish this objective, the decision was made to utilize the powerful catalytic asymmetric reduction process developed by Corey and his colleagues at Harvard.83 Treatment of 2-bromocyclohexenone (196) with BH3 SMe2 in the presence of 5 mol % of oxazaborolidine 197 provides enantiomeri-cally enriched allylic alcohol 198 (99% yield, 96% ee). Reductive cleavage of the C-Br bond in 198 with lithium metal in terf-butyl alcohol and THF then provides optically active (/ )-2-cyclo-hexen-l-ol (199). When the latter substance is treated with wCPBA, a hydroxyl-directed Henbest epoxidation84 takes place to give an epoxy alcohol which can subsequently be protected in the form of a benzyl ether (see 175) under standard conditions. 

Two approaches for the synthesis of allyl(alkyl)- and allyl(aryl)tin halides are thermolysis of halo(alkyl)tin ethers derived from tertiary homoallylic alcohols, and transmetalation of other allylstannanes. For example, dibutyl(-2-propenyl)tin chloride has been prepared by healing dibutyl(di-2-propenyl)stannane with dibutyltin dichloride42, and by thermolysis of mixtures of 2,3-dimethyl-5-hexen-3-ol or 2-methyl-4-penten-2-ol and tetrabutyl-l,3-dichlorodistannox-ane39. Alternatively dibutyltin dichloride and (dibutyl)(dimethoxy)tin were mixed to provide (dibutyl)(methoxy)tin chloride which was heated with 2,2,3-trimethyl-5-hexen-3-ol40. 

This procedure illustrates a general method for the stereoselective synthesis of ( P)-disubstitnted alkenyl alcohols. The reductive elimination of cyclic /3-halo-ethers with metals was first introduced by Paul3 and one example, the conversion of tetrahydrofurfuryl chloride [2-(chloromethyl)tetrahydrofuran] to 4-penten-l-ol, is described in an earlier volume of this series.4 In 1947 Paul and Riobe5 prepared 4-nonen-l-ol by this method, and the general method has subsequently been applied to obtain alkenyl alcohols with other substitution patterns.2,6-8 (I )-4-Hexen-l-ol has been prepared by this method9 and in lower yield by an analogous reaction with 3-bromo-2-methyltetra-hydropyran.10... 

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

Cyclopropene can also be used as the aUcene component and affords bicyclo[3.1.0]hexen-2-ones upon reaction with alkyne dicobalt octacarbonyl complexes in the presence of NMO (Scheme 250). Vinyl ethers and vinyl esters serve as ethene equivalents in Pauson-Khand reactions. For example, reaction of vinyl benzoate with complex (169) furnished cyclopentenone (170) (Scheme 251). This reaction was used in a synthesis of (-l-)-taylorine and nortaylorine. Allenes participate in intermolecular Pauson-Khand reactions affording alkylidene-substituted cyclopentenones (Scheme 252). ... 

For the synthesis of the trans-mmulsitQd 4a- p/-pumiliotoxin C 195 (Scheme 10.66), the imine prepared from galactosyl amine 150 and 5-hexenal was reacted with Danishefsky s diene to give the 2-substituted dehydropiperidinone derivative 196 with high diastereoselectivity. The subsequent addition of propyl cuprate promoted by borontrifluoride etherate furnished the 2,6-c/5 -disubstituted... 

The synthesis of disubstituted organo- em-bismetallics was also described by reaction of an ethereal solution of 1 -hexen-2-yllithium and allylmagnesium bromide in the presence of zinc bromide [116a] (Scheme 7-101). 

Attempted Synthesis of Specific Hexenyl Acetate Isomers. From Hexanal. Hexanal (5 ml. of 40% technical material) and 25 ml. 2-propenyl acetate were mixed, and 750 mg. p-toluene sulfonic acid was added. This solution was heated at reflux for 12 hours. The solution was then poured into 100 ml. water and extracted with two 50-ml. portions of ether. The combined ether extracts were washed with water, dilute aqueous sodium bicarbonate, and water and dried over sodium sulfate. After evaporating the ether, a gas chromatogram showed two major components present in a 3 2 ratio (probably the trans- and cis-hex-l-en-l-ol acetates, respectively). These components had identical retention times in the gas chromatograms to two of the major components present in the hexene-1 vinylation reactions. 

Eight- and nine-membered cyclic ethers are also generated via type-III reactions by treating the 5-hexen-l-ol acetals with Lewis acids. 8-endo Cyclization of 5-hexenyl methoxyethoxymethyl ether in the presence of two equivalents of tin(IV) chloride forms a 2 1 mixture of chlorinated and unsaturated 1 -oxycyclooctanes 11 and 1232. Vinylsilane undergoes an 8-endo Prins cyclization. After O-desilylation, oxocene 13, which is used as a precursor in a laurenyne synthesis, is obtained in 37% yield33. 

In an approach to the general synthesis of bicyclo[2,l,l]hexenes in which the double bond is introduced simultaneously with the construction of the basic ring system, the Ramberg-Backlund rearrangement of 2-chloro-3-thiabicyclo[3,l,l]heptane-3,3-oxides has been investigated. Thus (573), which is available in six steps from the cis + irons mixture of cyclobutane-1,3-dicarboxylic acids, is converted into the parent alkene (574) by reaction with sodium t-pentyloxide in dephenyl ether. 

An efficient procedure for the synthesis of 5-substituted dibenzofurans utilized the base catalyzed cyclization of 2-(6-substituted-3-(Z)-hexen-l,5-diyne)phenols. The intramolecular anionic cycloaromatiza-tion involved the addition of a hydroxyl function to the acetylene branch [330]. Thus heating dimethyl-terf-butylsilyl ether of 2-(6-butyl-3-hexene-l,5-diyne)phenol 3.700 with sodium methoxide in methanol under reflux for 16 hours afforded 5-butyldibenzofuran 3.701 in 60% yield following purification by column chromatography (Scheme 3.75) [330]. 

---