Spirocyclic furanones

Cyclization that eventualy places the resulting double bond endocyclic also occurs, as in the annulation of indoles and the formation of spirocyclic furanones and pyrrolones which involves a rearrangement process. ... 

The electroreductive cyclization of the furanone 118 (R = -(CH2)4CH=CH— COOMe Scheme 36) using a mercury pool cathode, a platinum anode, a saturated calomel reference electrode, and a degassed solution of dry CH3CN containing -Bu4NBr as the electrolyte, gave the spirocyclic lactones 119 and 120 in a ratio 1.0 1.1 (Scheme 37)(91T383). 

Beckmann rearrangement of 2,2,5,5-tetramethyltetrahydro-3-furanone oxime (362) afforded 1,3-oxazine 363 in 64% yield (equation 158). 1,4-Oxazines 365 were obtained by acidic deprotection-spirocyclization of oxime ethers 364 (equation 159) . 

In a recent approach to the preparation of dihydro-3(2H)-furanones by chemists from Thailand it was shown that consecutive, one-pot conventional treatment of 1,3-dithiane in THF with equimolar n-butyllithium, an epoxide, n-butyllithium, an aldehyde or ketone and, finally, methanesulfonyl chloride gave a spirocyclic dithioacetal in good (ca. 70%) yield. Hydrolysis with HgO/HgCl2 gave the corresponding dihydrofuranone. 

Examples for the transformation of 2-alkynyl-2-hydroxyalkanones to 3(2//)-futanones using either Pt(ll) or Au(lll) catalysts have been reported. This cycloisomerization process involves a 1,2-migration of an alkyl group. If cyclic substrates are employed, spirocyclic 3(2//)-furanones are obtained under ring contraction (Equation 51) <2006AGE5878>. 

The first case of an mrramolecular C-H carbenoid insertion was reported by Cane and Thomas in 1984 [12], with the special diazoacetate 17 forming the spirocyclic 5-lactone 18 in 45 % yield according to eq. (8). Doyle et al. recognized that this is a general methodology for the synthesis of y-butyrolactones [16], The reactivity of the C-H bond toward carbene insertion is increased in the vicinity of an ether functionality. Thus, the 3(2H)-furanone 20, as a useful building block in the total synthesis of (+)-muscarine, results in 40% yield from the diazo precursor compound 19 [17],... 

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