Furan formation carbonate with alcohol

Scheme 11.58 Furan formation of intramolecular reaction of propargylic carbonate with alcohol...

Fig (19) Octalin ketal (163) is converted to kete dithioacetal (164) by the cleavage of ketal function and condensation with carbon disulfide and methyl iodide. Subjection of (164) to the action of dimethylsulfonium niethylide and acid hydrolysis leads to the formation of unsaturated lactone (165).lts furan silyl ether derivative is caused to undergo Diets-Atder reaction with methyl acrylate to obtain salicyctic ester (166) which is converted by standard organic reactions toabietane ether (167). It is converted to aiiylic alcohol (168) by epoxidation and elimination. Alcohol (169) obtained from (168) yields orthoamide which undergoes transformation to amide (170). Its conversion to the previously reported intermediate has been achieved by epoxidation, elimination and hydrolysis. 

A related reaction occurs on irradiation of a hypochlorite (Scheme 2). The alkoxyl radical can again abstract a hydrogen atom in a l,S-shift, and the final chloro alcohol can be cyclized to form a tetrahydro-furan. Some processes lead directly to the cyclic product For instance, an alcohol with an accessible 8-hydrogen can be directly converted to a tetrahydrofuran on refluxing with lead tetraacetate. In a related reaction, treatment of an alcohol with silver carbonate and bromine can lead to the cyclic ether by initial formation of a hypohalite. The cyclization occurs when the rearranged radical is converted to a cation, either by oxidation with Pb(OAc)4 or by silver-assisted loss of halide ion. Hypoiodites are also fiequent-ly used, generated in situ. ... 

Aldehydes can be converted directly into 0,Se-acetals and used for radical cyclizations. This procedure is equivalent to a ketyl radical cyclization process, a typical example is shown in Eq. (13) [31]. Acetals such as 54 can also easily be converted into 0,Se-acetals by treatment with (z-Bu)2AlSePh. The selenide 55 is used for an efficient radical carbon-carbon bond formation [Eq. (14)1 [32]. A very reliable route to 0,Se-acetals from alcohols through the corresponding (tributylstannyl)methyl ethers has been used for the preparation of tetrahydro-furan derivatives [33]. 

The next and particularly crucial phase of the synthesis was the construction of the quaternary carbon center associated with galanthamine as well as the formation of the furan or B-ring. While it took some time to establish the right sequence of reactions to realize such an outcome, this was eventually achieved in just three steps (Scheme 4). The first was the engagement of the allylic alcohol moiety within compound 15 in an Eschenmoser-Claisen (EC) rearrangement by treating it with the dimethyl acetal of... 

Heteroatom Wittig chemistry also includes reactions of N-sulfonyl imines. It was demostrated that these compounds underwent olefination reactions with nonstabilized phosphonium ylides under mild conditions to afford an array of both Z- and E-isomers of 1,2-disubstituted alkenes, allylic alcohols, and allylic amines.Additionally, studies of the reactions of 5-bromo-4,6-dimethyl-2-thioxo-l,2-dihydropyridine-3-carboni-trile and thiazolidinone with phosphorus ylides have proved the formation of new phosphonium ylides. Annulations via P-ylides are a common occurrence in the literature. For example, on photochemical irradiation, phosphonium-iodonium ylides were shown to undergo 1,3-dipolar cycloaddition reactions with triple bonds, via a carbene intermediate, to yield furans. " Even more common are the reactions of Morita-Baylis-Hillman (MBH) acetates and carbonates. Zhou et al. demostrated that these substrates were able to generate very reactive 1,3-dipoles in the presence of tertiary phosphines the dipoles then underwent cycloaddition reactions to yield annulation products (Scheme 16). ... 

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