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Cis-Trans Enantiomerism, Fulvene Cycloadditions And View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Electronic Theses and Dissertations at Indian Institute of Science SYNOPSIS ‘Topics in Synthetic Methodology: from Heterocycles to Hydride Transfers’ This thesis, largely describing diverse studies in organic synthesis, is divided into three parts. Part I, titled ‘Heterocycles’, describes in two chapters studies directed towards elaborating certain thiazole and oxazole derivatives as useful synthons. Part II, titled ‘Hydride transfers’, describes in two chapters synthetic and some mechanistic studies involving the Cannizzaro and Tishchenko reactions, apart from work with chirally-modified alumino and borohydride reagents. Finally, Part III, titled ‘Miscellaneous studies’, describes structural studies on cyclic carbonates. PART I. HETEROCYCLES O O S O DCC, 10min. (i) (ii) R OH S S Ph N N S H DCM, rt N N O Ph Ph Ph 1 I 2 (68-81%) (i) 5 mol % Pb(OAc)2Ph(OH)2 (ii) RCHO, DCM, rt, 20min. R = Et, n-Pr, i-Pr, i-Bu, n-Hexyl, Ph, PhCH=CH-, 4-MeO-C6H4- Scheme I Chapter 1. The Erlenmeyer synthesis with a thioazlactone and its applications. This chapter describes the synthesis and reactivity of 2-phenylthiazolin-5-one (1). This was generated in situ from N-(thiobenzoyl)glycine and N,N’-dicyclohexylcarbodiimide in CH2Cl2/r.t., and condensed with various aldehydes (Scheme I). The reaction occurred rapidly at room temperatures to furnish the corresponding 4-alkylidene or arylidene products (2) in excellent yields (68-81%). i The reaction was catalysed by basic lead acetate, and was equally successful with both aliphatic and aromatic aldehydes. These reactions define a remarkably mild version of the classical Erlenmeyer azlactone synthesis,1 which is apparently enabled by the greater aromaticity of the intermediate thioazlactone anion (I, relative to the corresponding azlactone anion).2 These studies considerably extend the scope of the original Erlenmeyer strategy (particularly to include aliphatic aldehydes), which remains an important method for the synthesis of α-amino acids. Chapter 2. 5-Methoxyoxazole as a synthon for α-amino acids via ‘ortho-metallation’. This chapter describes the preparation of 5-methoxy-2-phenyloxazole (3), its deprotonation at C4 and the electrophilic reactivity of the resulting anion (II) (Scheme II). The above deprotonation of 3 was expected to be assisted by the 5-methoxy group, in analogy to the well-established ortho- metallation strategy with aromatic substrates such as anisole, etc.3 R H B N B-M N N M R-X N M Ph Ph OMe O OMe Ph O O Ph O OMe O Me 3 II (+BH) 4 (B-M = nBuLi + TMEDA/THF/-78 oC (R = D, Me) or LDA + HMPA/THF/-78 oC) R R OH N 6N HCl II + RCHO PhCON CO2H 6h Ph O OMe H 6 5 R = Ph (52%), i-Pr (78%) Scheme II Although oxazole 3 was unreactive towards n-butyl lithium or lithium diisopropylamide (nBuLi or LDA, at -78 oC), the desired deprotonation was accomplished by the addition of an equivalent ii of N,N,N',N'-tetramethylethylenediamine or hexamethylphosphoramide (TMEDA or HMPA, Scheme II). The resulting formation of II was indeed evidenced by the quenching of the reaction mixture with D2O, and the isolation of the 4-deutero derivative of (4, R = D) in 75% yield. The methylation of II with MeI (THF/r.t.) furnished the expected 3-methyl derivative (4, R = Me) in excellent yield (63%). However, alkylation of II with reagents bearing longer alkyl chains (C2 and above) failed, despite repeated attempts under a variety of conditions. (This is possibly due to a combination of steric and electronic factors.) All the same, II could be added to several aldehydes to obtain the corresponding carbinols (5) is good yields. Carbinols 5 on treatment with acid, underwent ring cleavage to the open chain amides 6. PART II. HYDRIDE TRANSFERS Chapter 3. New approaches to the Cannizzaro and Tishchenko reactions. These studies were based on interesting insights into the mechanism of the Cannizzaro reaction. This redox reaction generally occurs in aqueous base, and converts non-enolisable aldehydes (7) to a mixture of the corresponding alcohol (8) and carboxylic acid (9) (Scheme III). Mechanistic studies have revealed a hydride transfer mechanism, the reaction being of high kinetic order: second order in the aldehyde substrate and first or second order in base.4 These indicate that the reaction would be exponentially speeded up at high concentrations, and that minimal amounts of solvent should be employed. Also, the fact that the Cannizzaro reaction is preferably performed in water indicates that the hydrophobic effect5 possibly plays a role. iii (i) 2ArCHO ArCH2OH + ArCO2H (ii) 2PhCHO PhCO2CH2Ph 7 8 9 10 (70%) (75-95%) (ii) cat. NaOMe/THF/rt/16h (i) 4 eq. KOH-H2O (satd.)/rt/30-60 min. Scheme III Indeed, the Cannizzaro reaction of several aromatic aldehydes (7) could be conveniently effected with four molar equivalents of potassium hydroxide, employed as a saturated aqueous solution, within two hours at room temperatures. The reaction was particularly rapid in the case of the activated nitro substituted benzaldehydes.6 Previous procedures involved overnight reaction and a difficult work-up because of the emulsification of the reaction mixture.4 (It was also observed that the reaction with benzaldehyde was almost completely suppressed at high ionic strengths, possibly evidencing the above hydrophobic effect.) The Tishchenko reaction is an interesting variant of the Cannizzaro reaction,4,7 in which an alkoxide ion nucleophile adds initially to a benzaldehyde, to produce finally the corresponding alkyl benzoate (Scheme III). In this study, the Tishchenko reaction was carried out with benzaldehyde and catalytic amounts of sodium methoxide, again at high concentrations in tetrahydrofuran solution. These conditions lead to the formation of benzyl benzoate (10) in excellent yields (Scheme III).6,8 The initially formed methyl benzoate and benzyl alcoholate react further to form benzyl benzoate, thus regenerating methoxide ion. However, the reaction essentially failed with substituted benzaldehydes, and was also decelerated at higher pressures (presumably due to a decrease in the diffusion of the reactive species.) iv (i) O O O O OH O Q M H 11 H (i) LiAlH4 or NaBH4 III Q=Al, B; M=Li, Na Scheme IV Chapter 4. Enantioselective reductions of prochiral ketones by chirally modified alumino-and borohydrides. The preparation of chiral hydride reducing agents is a topic of much current interest.9 In this study, an attempt was made to prepare aluminohydride and borohydride derivatives of (1S)-(+)-ketopinic acid (11), and employ them for enantioselective reduction of prochiral ketones (Scheme IV). Thus, ketopinic acid was allowed to react with a molar equivalent of either lithium aluminum hydride or sodium borohydride; however, the resulting species (presumably III) reduced acetophenone with very low enantiomeric excess (~ 10%). Modifications, e.g. with the ethyl ester of 11, led to similar results. The reasons for the observed low e.e. could be many, such as poor enantioface discrimination, disproportionation of the reagent to afford achiral reducing species, etc. However, in view of the low e.e. values, the study was not continued further. In a separate study, a method for the selective reduction of aldehydes over ketones, by reaction with NaBH4/diglyme was developed (not shown). However, it was abandoned as a similar report appeared in the literature during the study.10 v Part III. MISCELLANEOUS STUDIES O O F R R' NO O O 2 (i) NO2 R R' NO2 (i) K2CO3-H2O/rt/12h NO2 12 13 (R,R' = Me, Ph, OMe) (30-80%) Scheme V Chapter 5. Nucleophilic substitution of 2,4-dinitrofluorobenzene (DNFB) under aqueous conditions: a route to substituted indoles. The reactions of DNFB (12, Scheme V) with various heteroatom centred nucleophiles in water are very well known, particularly its utility in peptide sequencing by the Sanger methodology.11 It was of interest to extend these studies to the case of carbon-centred nucleophiles, particularly under aqueous conditions. DNFB was indeed found to react with highly acidic C-H acids (Scheme V), particularly acetylacetone (80%), benzoylacetone (57%) and acetoacetic ester (~ 30%), under weakly basic aqueous conditions. The products were derived from nucleophilic substitution of the fluoride ion by the conjugate base of the C-H acid (cf. 13). The yields with diethyl malonate (22%) were very low, and the reaction failed with several other substrates, such as Meldrum’s acid, cyanoacetic ester, barbituric acid, etc. It appears that the reaction depends on a complex combination of effects, with the acidity of the C-H acidic substrate being possibly dominant. Although the yields were variable, it is noteworthy that the products afford entry into several substituted indole derivative by known methodology. vi MeO2C O O O MeO2C 14 Scheme VI Chapter 6. Crystallographic studies of cyclic carbonates of dimethyl tartrate. In the course of a stereochemical study of tartrate derivatives (aimed at discovering conglomerate behaviour), several cyclic carbonates of the isomers of dimethyl tartrate were prepared and subjected to X- ray diffraction crystallographic analysis. It was discovered that the cyclic carbonate (14, Scheme VI) derived from dimethyl D-tartrate existed in two polymorphic modifications (not shown). The lattice packing features for the two polymorphs were different. However, polymorphism was not established in the case of the enantiomeric L-tartrate derivative. The DL form of 14 was found to be a racemic compound rather than a conglomerate. References and notes 1. Rosen, T. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Heathcock, C. H., Eds.; Pergamon: Oxford, 1991; Vol. 2, pp 402-407. 2. Chandrasekhar, S.; Karri, P. Tetrahedron Lett. 2006, 47, 5763-5766. 3. Snieckus, V. Chem. Rev. 1990, 90, 879-933. 4.
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