Aldehyde quenching

LDA metalation of the 2-carbamate 344 followed by aldehyde quench afforded mixtures of products 347 and 348 (Scheme 106). The formation of the carbostyryl derivative 348 can be rationalized by an intramolecular carbamoyl migration (349- 350) (Scheme 107) driven by the leaving group ability of the 2-carbamoyl group. 

LDA is totally unsuitable for the generation of enolates from aldehydes quench reactions of the resulting mixtures with chlorotrimethylsilane gave only unidentified viscous products [4]. 

PET suitable for bottle manufacture is produced by a modified process. Here the high-viscosity polymer melt is subjected to a rapid quenching in water to produce clear amoiphous pellets. These are further polymerised in the solid phase at temperatures just below the T . This is useful to reduce aldehyde content, since aldehyde-forming degradation reactions occur less in the lower temperature solid phase polymerisations. Aldehydes can impart a taste to beverages and it is important to keep the aldehyde content to below 2.5 p.p.m. 

To a solution of 0 01 mol of lithium hexamethyldisilazide and 0 01 mol of HMPA dissolved in 50 mL of anhydrous THF at 78 °C was added 0 5 g (0 004 mol) of l-fluoro-3,3-dimethylbutanone m THF over 1 mm To the solution of the enolate was then rapidly added 0 003 mol of the aldehyde m THF After stimng an additional 2 mm, the reaction was quenched by rapid addition of a saturated ammonium chlonde soluuon Extractive workup with hexanes yielded on evaporation the product as a clear colorless oil isolated yield... 

The procedures for the reactions of the boronate tartrate esters and tartramides are similar71 13. The reactions of tartramides are much slower (typically 2 4 days) due to their poor solubility in toluene at —78 °C, and these reactions were therefore quenched at —78 °C by the addition of excess sodium borohydride in ethanol to consume any unreacted aldehyde before the reactions were allowed to warm above — 78°C73. 

A concentrated solution of the 3-phcnylsulfinylalcohol in THF (2 mL THF per 0.5 g of sulfinylalcohol) cooled to — 78°C is added to a solution of 2.1 equiv of LDA in THK (10 mL per 3 mmol of L DA). After stirring for 1 min 1.5 equiv of the aldehyde are added to the reddish solution and one minute later, the reaction is quenched with sat. aq NH CI. The mixture is diluted with water, extracted with ElOAc or CI1CI3 and worked up. The product is purified by column chromatography. 

To a solution of 0.6 g (2.8 mmol) of A -sulfinyl-4-toluenesulfonamide in 10 mL of dry CH,C1Z is added a solution of 2 mmol of aldehyde 1 in 2 mL of dry CI12CI2 at r.t. under argon protection. This mixture is stirred for 2-3 h (exception R1 = /-Pr, 48 h) before 6 mmol of the Grignard reagent is added at 22-30 C. After stirring for another 60 min the reaction is quenched with 50 mL of sat. aq NaCl and twice extracted with HtOAc. The combined extracts are dried over MgS04 and concentrated. Chromatography of the crude product on silica gel (pet. ether/EtOAc 2 1) affords 3a and 3b. 

An analogous study concerned with the synthesis of prostaglandin analogs was also highly diastereoselective. The initial anionic adducts were quenched with various aldehydes, however, the diastereoselectivity at the carbinol stereocenter was not reported21. 

The use of enantiomerically pure (R)-5-menthyloxy-2(5.//)-furanone results in lactone enolates, after the initial Michael addition, which can be quenched diastereoselectively trans with respect to the /J-substituent. With aldehydes as electrophiles adducts with four new stereogenic centers arc formed with full stereocontrol and the products are enantiomerically pure. Various optically active lactones, and after hydrolysis, amino acids and hydroxy acids can be synthesized in this way317. 

Alternatively (27), when the same procedure was followed using THF containing 5eq. HMPA (CAUTION—CANCER SUSPECT AGENT) (relative to the aldehyde) at —78°C for 15-20s, followed by quenching at —78 °C and isolation, the ( )-enynes were obtained, ( ) (Z) 20 1 to 10 1, 60-64%. 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

---