Reaction with aldehydes or ketones

Cycloaddition of COj with the dimethyl-substituted methylenecyclopropane 75 proceeds smoothly above 100 °C under pressure, yielding the five-membered ring lactone 76. The regiocheraistry of this reaction is different from that of above-mentioned diphenyl-substituted methylenecyclopropanes 66 and 67[61], This allylic lactone 76 is another source of trimethylenemethane when it is treated with Pd(0) catalyst coordinated by dppe in refluxing toluene to generate 77, and its reaction with aldehydes or ketones affords the 3-methylenetetrahy-drofuran derivative 78 as expected for this intermediate. Also, the lactone 76 reacts with a, /3-unsaturated carbonyl compounds. The reaction of coumarin (79) with 76 to give the chroman-2-one derivative 80 is an example[62]. 

The primary and secondary alcohol functionahties have different reactivities, as exemplified by the slower reaction rate for secondary hydroxyls in the formation of esters from acids and alcohols (8). 1,2-Propylene glycol undergoes most of the typical alcohol reactions, such as reaction with a free acid, acyl hahde, or acid anhydride to form an ester reaction with alkaU metal hydroxide to form metal salts and reaction with aldehydes or ketones to form acetals and ketals (9,10). The most important commercial appHcation of propylene glycol is in the manufacture of polyesters by reaction with a dibasic or polybasic acid. 

Mixtures of anhydrous hydrogen fluoride and tetrahydrofuran are successfully used as fluorinating agents to convert 1,1,2-trifluoro-l-allcen-3-ols, easily prepared from bromotrifluoroethene via lithiation followed by the reaction with aldehydes or ketones, to 1,1,1,2-tetrafluoro-2-alkenes The yields are optimal with a 5 1 ratio of hydrogen fluoride to tetrahydrofuran The fluorination reaction involves a fluonde lon-induced rearrangement (Sf,j2 mechanism) of allylic alcohols [65] (equation 40)... 

A conversion typical of a-halo-a-lithioaUcanes is the formation of epoxides that results from their reaction with aldehydes or ketones. As illustrated in equation 61, the bromo-lithium carbenoid is usually generated by halogen-lithium exchange. The intermediate lithium aUcoxide 113 undergoes an in situ ring closure to give the oxirane 114 . 

These furanes can be used for synthesis of bislactones or y-hydroxybutenolides by reaction with aldehydes or ketones in the presence of TiCl4 (equations II and III). 

Access to the imino group of oxazolidine-4-carboxylic acid is even more sterically hindered when substitutions are performed at the C2 position. For such cases, the typical procedure is based on the conversion of the desired serine or threonine dipeptides with unprotected hydroxy groups into the oxazolidine rings by reaction with aldehydes or ketones,1139,1691 as described in Vol. E 22a, Section 2.3.2.4. 

Aldol reactions.1 The anion generated (BuLi) from chromium carbene complexes undergoes aldol reactions with aldehydes or ketones activated by a Lewis acid. Best results are obtained with ketones in the presence of BF3 etherate, whereas TiCl4 is the preferred catalyst for aldehydes and acetals. 

Ketene dithioketals,1 The reagent undergoes Wittig-Horner reactions with aldehydes or ketones to give 2-alkylidene-l,3-dithianes in 80-100% yield. The corresponding P=S analog reacts with aldehydes in only moderate yield (25-65% yield). 

One application of this catalytic generation of homoenolate type intermediates is in the stereoselective formation of y-butyrolactones 64 from a,/ -unsaturated aldehydes 62 and their reaction with aldehydes or ketones 63 [60]. (For experimental details see Chapter 14.19.2). Glorius [60a] and Bode [60b] almost simultaneously published their results utilizing a N-heterocyclic carbene generated from a bisar-ylimidazolium salt 65 (IMes). The corresponding disubstituted y-butyrolactones... 

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