Frater-Seebach alkylation

Sand-sodium base. See Sodium-sand base a-Santalene, 42 Sobering process, 118-119 Schiff bases. See Imines 9,10-Secosteroids (calciferols and tachysterols), 31-32, 34, 281-283, 289 Seebach-Frater alkylation, 27. AT-Selectride = potassium hydrotris(l-mcthylpropyl)borate(l —) (KBHBVl), 107 XS-Selectride 1 — potassium tris(l,2-dimethyl-propyl)hydroborate(l —) (KBHSiay), 107 Z-Selectride = lithium hydrotri (l-... 

Butanoic acid, 3-hydroxy-, esters pr, 176 Frater-Seebach 2-alkylation, 27 —, 3-oxo-, esters (acetoacetic esters) pr., 176 carbon acidity, 10 carboxymethyl da-synthon, 19, 207 dianion 4-alkylation, 24, 207,325-326... 

Alkylation of aldol type educts, e.g., /1-hydroxy esters, using LDA and alkyl halides leads stereoselectively to erythro substitution. The erythro threo ratio of the products is of the order of 95 5. Allylic and benzylic bromides can also be used. The allyl groups can later be ozonolysed to give aldehydes, and many interesting oligofunctional products with two adjacent chiral centres become available from chiral aldol type educts (G. Frater, 1984 D. Seebach, 1984 see also M. Nakatsuka, 1990, p. 5586). 

The synthesis of the C20—C26 fragment started with a 4-alkylation of methyl aceto-acetate The first stereocentre was introduced by enantioselecuve catalytic hydrogenation with Noyort s (S)-binap rhodium complex (cf p 102f.) Stereoselective Frater-Seebach alkylation with allyl bromide introduced the second stereocentre in 90% yield (cf p 27) Stereospecifid introduction of the stereocentres C24 and C2 was achieved by a chelation controlled addition of an allylstannane to an aldehyde (see p 66f) After some experimentation with Lewis acid catalysts and reaction conditions a single diastereomer of the desired configuration was ob-... 

Chiral P-hydroxy esters and l,3-dioxan-4-ones are well-known substrates for diaster-oselective a-alkylation reactions developed by Frater [20] and Seebach [21]. These chiral compounds are available in both enantiomeric forms, and have been also ami-nated at the a-carbon with high stereoselectivity. 

Fragmentation reactions, 89-90, 141-142 Frater-Seebach alkylation, 27, 325-326 Friedel-Crafts acylation ... 

---