Ring expansion, acid catalyzed rearrangement

Maltol. Otsuka Chemical Co. in Japan has operated several electroorganic processes on a small commercial scale. It has used plate and frame and aimular cells at currents in the range of 4500—6000 A (133). The process for the synthesis of maltol [118-71 -8], a food additive and flavor enhancer, starts from furfural [98-01-1] (see Food additives Flavors and spices). The electrochemical step is the oxidation of a-methylfurfural to give a cycHc acetal. The remaining reaction sequence is acid-catalyzed ring expansion, epoxidation with hydrogen peroxide, and then acid-catalyzed rearrangement to yield maltol, ie ... 

Hydroxymethyltetrahydrofuran is converted into 5,6-dihydrcM-//-pyran and the acid catalyzed rearrangement of other 2-substituted tetrahydrofurans forms 3-hydroxytetrahydropyrans (Scheme 27). Thiepan-4-one results from ring expansion of tetrahydrothiopyran-4-one (Scheme 28). 

From four-membered rings An acid-catalyzed transformation has been observed in the conversion of l-[l-methylsulfinyl-l-(methylthio)alkyl]cyclo-butanol to 3-methyl-2-(methylthio)cyclopentanone [9]. - Rearrangement of a /3-lactone to a y-lactone derivative in the presence of magnesiumdibromide [10]. - A borontrifluoride catalyzed cyclobutene to cyclopentene rearrangement [11]. - Ring expansion of a [2+2] photoadduct to a five-membered ring [12]-... 

An interesting method for the preparation of six-membered heterocycles involves ring expansion of smaller rings. One example is 1,4,2-dioxazine 254, which was prepared via acid-catalyzed rearrangement of benzofurandione dioxime 253 (Equation 73) <2006GEP2006021368>. 

The chiral phosphoric acids (127, 132, 137, 154, 198, 199) catalyzed an asymmetric ring expansion-type, semipinacol rearrangement reaction affording chiral spiroethers (197) from (198) in up to 98% ee and good to high chemical yields under mild conditions, has been discovered by Tu and co-workers (Scheme 55)7 ... 

A number of examples of acid catalyzed ring expansion of acyl and thioacyl azetidines to sbc-membered rings have been reported (B-73MI50903). This type of rearrangement (Scheme 2) is similar to the more general vinylaziridine to pyrroline ring expansion. 

Acid-catalyzed ring expansions of related 6-5-4 tricyclic systems are also known. For example, the acetal (257), formed by cycloaddition of keten acetal and 3-ethoxyisoindolone, in hydrochloric acid rearranges to the 2-benzazepine-l,5-dione (258) (75JA7288). 

Lewis acid catalyzed ring expansion is found for 2-azabicyclo[3.2.0]heptane-5-carboxylates with an imidate group in the five-membered ring, e.g. 978 which rearranged to give 10.79,80... 

Base-catalyzed ring-expansion of benzisoxazolones (217) provides a general route to 2,2-dialkyldihydro-l,3-benzoxazin-4-ones (218) (78CPB549) and the 2,4-dione (220) can be prepared by a Beckmann rearrangement of the oxime (219) (02CB3647). 2-Thion-4-ones (222) are formed from salicylic acids (221) by treatment with triphenylphosphine diisothiocyanate in dichloromethane at -40 °C (78CI(L)806). 

A synthesis of cycloheptanone (5) starts with the base-catalyzed addition of nitromethane to cyclohexanone " the procedure calls for a ratio of 3.25 moles of nitromethane to 2.5 moles of ketone. The sodium enolate (2) separates and is collected and iicldifled to liberate the nitro alcohol (3), which Is hydrogenated to the amino alcohol (4). Nitrous acid then elfects deamlnatlve rearrangement with ring expansion to c) cluheptanone (5). 

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