New C-4-chiral 1,3-thiazolidine-2-thiones: excellent chiral auxiliaries for highly diastereo-controlled aldol-type reactions of acetic acid and .alpha.,.beta.-unsaturated aldehydes

Abstract

Scheme I 4 concomitantly, 2-methylene ketone 4g was formed (monitored by NMR). After 4 h, dehydrostannation was complete, and the precipitate was filtered off. Aqueous workup of the filtrate (pentane/aqueous NH4C1) followed by evaporation of the solvents and bulb-to-bulb distillation afforded 2-methylenecycloheptanone (4g) in 80% yield. Similarly, essentially quantitative conversion of 3 to 4 (eq 1) was observed in the reaction of 3 with Me2SO-d6/CDCl3 3 4 at 60 °C (by NMR) except for 3f which did not bear hydrogen ß to the stannyl group.9 The results are shown in Table I. The dehydrostannation of 3d,e,g,h proceeded with shorter reaction time than 3a-c. It seems that for dehydrostannation to occur, the intramolecular coordination of the carbonyl oxygen to the Sn atom (as indicated by IR) may play a key role. In fact, dehydrostannation did not take place with simple n-BuSnCl3 even under more forcing conditions (60 °C, 1 day). One plausible explanation for the present reaction is shown in Scheme I. The initial role of Me2SO may be the one as a base toward /3-hydrogen10 which may be sufficiently acidic due to the inductive effect of the adjacent carbonyl coordinated to the Sn. Thus, the deprotonation from 5 and subsequent destannylation via 6 would occur to lead to 4 and HSnCl3(Me2SO)" complex.11,12 Interestingly, the reaction of siloxycyclopropane 7 with SnCl4 in CH2C12 proceeded similarly and afforded ß-tríchlorostannyl aldehyde 8 (semisolid, vc=0 1680 cm-1). The (9) As the preparative method of /3-stannyl ketones 3, hydrostannation of ,/8-unsaturated ketones with HSnClg (in situ generated under acidic conditions: HC1 and SnCl2) has been known. The observed dehydrostannation promoted by Me2SO formally corresponds to its reverse reaction. See:

Isolation and Total Synthesis of Icumazoles and Noricumazoles—Antifungal Antibiotics and Cation‐Channel Blockers from Sorangium cellulosum

“…= 1:1). [18] Methyl ester 16 was elaborated to the vinyl iodide 20 by reduction to the corresponding aldehyde followed by Seyferth-Gilbert homologation [19] using Bestmanns reagent. [20] The resulting alkyne was subjected to a syn-hydrozirconation [21] followed by iodination.…”

Exploiting Ruthenium Carbene-Catalyzed Reactions in Total Synthesis of Marine Oxacyclic Natural Products

“…To minimize the number of transformations, we began our synthesis with transcinnamaldehyde (54), whose double bond was eventually exploited in a late-stage RCM ( Figure 9). A four-step sequence, including a Nagao asymmetric acetate aldol reaction 54 and an EvansTishchenko reduction, 55 delivered the homoallylic alcohol 55. Selective homologation of the terminal double bond without touching the styryl group was achieved by means of a H-bonding-directed olefin cross-metathesis.…”

Synthesis of Macrocyclic Lactams and Lactones Containing Sulfur and Nitrogen

“…[25] Ретросинтетический анализ молекул туггацинов 41-43 предполагал образование их из двух блок-синтонов 44 и 45. [27] Конвергентный синтез первого из них осуществлен из двух хиральных непредельных соединений: кислоты 46, полученной последовательными реакциями альдольной конденсации с акролеином по Нагао [28] и Эвансу, [29] и амина 48 -продукта реакции олефинирова-ния по Виттигу доступного альдегида 47 [30] илидом 51. Формирование тиазольного фрагмента проведено в две стадии: сначала происходит образование амидной связи между фрагментами 46 и 48 в присутствии тетрафторбората O- (1H-бензотриазол-1-ил)-N,N,N',N'-тетраметилмочевины (TBTU), а затем (после удаления защитной группы) -циклизация под действием молиб-денового катализатора 52.…”