Diastereoselectivity, from

Double deprotonation of tetrahydro-2-(2-nilroethoxy)-2//-pyran (8) and reaction with electrophiles provides a variety of substituted and functionalized nitroaldols1 Reaction with aldehydes affords 2-nitro-l,3-alkanediols 9 in 44 90% yield and high diastereoselectivities. From analogy of their II-NMR spectra and comparison with known compounds, the (R, R ) relative configuration is likely15. 

Tetraene 141 has been converted into various complex polycondensed adducts by reacting with a variety of dienophiles such as maleic anhydride, N-phenylmaleimide, N-phenyltriazolinedione,p-benzoquinone and tetracyano-ethylene carried out under thermal conditions. All cycloadditions occurred facial-diastereoselectively from an outside attack and provided monocycloadducts which had an exceptionally close relationship between diene and dieno-phile and then underwent intramolecular cycloaddition [125]. The reaction between 141 and p-benzoquinone is illustrated in Scheme 2.53. 

Cycloaddition reactions of vinyl trimethylsilane with C -glycosyl nitrones gave moderate to good yields (67%-74%). Estimation of diastereoselectivities from isolated yields showed total endo preference for the reaction of the D-galacto nitrone. High endo preference was observed for the D-ribo analog, but exo preference for the D-xylo one (814). 

The stereoselectivity proved to be dependent on the nature of the halide ligand X and was found to be complete for X = Br. Reversal of the diastereoselectivity from anti to syn has been accomplished by adding Et2OBF3 [37]. The predominant syn selectivity in the presence of this Lewis acid is consistent with a non-cyclic antiperiplanar transition state. 

The enolization (LDA, THF) and subsequent condensation of a-amino ester 53a under kinetic conditions affords low levels of kinetic aldol diastereoselection. From the preceding discussion it is probable that the major enolate derived from 53a possesses the E)-geometry. The observation that moderate levels of erythro diastereoselection are observed with benzaldehyde (Table 16) are consistent... 

These intrazeolite singlet oxygen ene reactions have synthetic potential because the cis effect observed in solution is suppressed in the zeolite [13]. Consequently, allylic hydroperoxides which are inaccessible by other routes may be available via this new technology. For example, photo-oxidations of aryl-substituted alkenes, 7, in sensitizer-doped NaY react to generate the allylic hydroperoxides as the major or exclusive product [17]. In contrast, in solution, the hydroperoxides are formed in only 5-20% yields, with 2-1-2 and 4-1-2 adducts dominating the reaction mixtures. In the case of 2-methyl-5-phenyl-2-hexene, 8, the regio-selectivity for 8b and 8c improved from 47% to 94% and the diastereoselectivity from 10% to 44% as the reaction is moved from solution into the zeolite [18] ... 

Iminomethyldithiolane 7 can be deprotonated with potassium hexamethyldisilazide in THF at — 78 °C and alkylated to give 8 in good yield and with high diastereoselectivity. From 8, a-aminoalkylphosphonic acids 5 are easily obtained by mild acidic hydrolysis. 

The enolates of l,3-dioxolan-4-ones 1 and l,3-oxathiolan-4-ones2, which are y-lactones substituted with a heteroatom, can be prepared using normal conditions (LDA, THF) and subsequently alkylated. If one takes, e.g.. a nonracemic a-hydroxy acid such as lactic acid 3, then on formation of the heterocycles 1 (R1 = CH3 R2 = alkyl) chirality is transferred from the a-po-sition (C-5) in the starting acid to C-2 in 1, forming (2R,5R)-4 and (2S,5R)-5. If 4 and 5 are easily separable or, even better, if one of them is formed diastereoselectively from 3 then 4 and/or 5 can be transformed to the respective enolates 6 and 7 and alkylated with R X. 

After reaction of the excess lithium with isoprene the enolate is alkylated with allyl bromide diastereoselectively from the less hindered face, opposite to the axial methyl group at the bridge head, to provide allyl ketone 3 as the single diastereomer. 

Concellon, J. M. Bardales, E. Synthesis of (E)-a,/ -unsaturated esters with total or high diastereoselectivity from a,/ -epoxyesters. Org. Lett. 2002, 4, 189-191. 

The paper reports a highly efficient Claisen rearrangement of a (Z)-silyl ketene acetal (equation I) by use of DMPU to control the stereoselectivity. Use of HMPA lowers the diastereoselectivity from 96 to 84% de. 

In the additions to 45 it was noted, through variations of the stoichiometric loading of Lewis acid, that amounts of InCl3 less than 2 equiv resulted in lower diastereoselectivity. From this it may be inferred that the Lewis acid, aside from its usual chelation by the /V-acylhydrazone. may interact with another Lewis basic site (e.g., the ester). 

The organolithium 148, formed diastereoselectively from 147 on deprotonation with s-BuLi-TMEDA, is configurationally stable at -78 °C, but over a period of hours at -45 or -30 °C slowly epimerises.70... 

Hydroxy alkenyl and alkynyl nitriles can be subjected to a one-pot addition-alkylation sequence, leading to polysubstituted products. In the case of open-chain systems, diastereoselectivity from 2.3 1 to 6.6 1 was achieved. Cyclic systems may lead to a highly diastereoselective reaction.234,234a 234c iv-Chloroalkenylmagnesium bromides give bicyclic products (Scheme 77 ) 235>236... 

The more recent work on this area deals predominantly with the asymmetric induction in aza Diels-Alder reactions in order to develop a novel powerful tool for the stereoselective synthesis of biologically active compounds. Thus, Wald-mann et al. demonstrated the utility of chiral imines derived from enantiopure amino acids by obtaining the cycloadduct 3-3 in very good diastereoselectivity from imine 3-1 and Brassard s diene 3-2 (Fig. 3-1) [181]. 

The facial diastereoselectivity derived from-the ratio (3 + 4)/(5 + 6) was 50%, while the exo/endo selectivity derived from the product ratio (3 + 5)/(4 4- 6) was 40%. Oxetanes 9a,b were obtained with a low diastereoselectivity from the reaction of (R)-isopropylideneglyceraldehyde 7 with 3,4-dimethylfuran 8 [6]. Oxetanes 9a,b have been used for the synthesis of asteltoxin. Enantiopure acyl cyanides were used in the same way as chiral carbonyl reaction partners [7] and camphor for the addition with electron-poor alkenes like dicyanoethylene [8]. In the latter case the reaction occurs in the S i state of the carbonyl compound. 

The reaction has been used to synthesize libraries of benzonaphthyridines 196, in high diastereoselectivity, from the cycloaddition of 1,4-dihydrop3Tidines with imines formed from aldehydes and anilines. When cyclic enol ethers were used as dienophUes, mixtures of diastereomers 197 were obtained. These compounds were oxidized to the corresponding quinolines 198 and were further transformed to the quinolinium salts 199 as shown in Scheme 36 [76]. Compounds 196 and 198 were tested for their ability to inhibit human propyl oligopeptidase (POP) and were found to have modest potencies. Much better results were obtained when the quinoline nitrogen was methylated to provide adducts 199. The cationic center improved the inhibitory activity of these compounds (Fig. 23). 

Only one diastereomer 1 was produced from ( )-l,2-diphenylethylene and, although with lower yield and diastereoselectivity, from (Z)-l,2-diphenylethylene via radical addition. 

However, even the a-unsubstituted y- and -alkenyl oximes readily give the corresponding five-and six-membered cyclic nitrones when the alkene is activated by a terminal ester group. For example, the bicyclic nitrone 4 was obtained with moderate diastereoselectivity from 3 via the oxime formed in situ1. From the optically active menthyl esters 5,1 1 mixtures of diastereomer-ic nitrones 6 were obtained. 

Bisadducts, e.g., 11-14 are obtained from tetraenic propellanes by reaction with two equivalents of the dienophile, generally with complete diastereoselectivity. From the syn.syn-bisadducts 13 and 14 some cage molecules have been prepared7,8-1 12,15-21. 

We have found that the BINOL-derived titanium complex serves as an efficient catalyst for the Mukaiyama-type aldol reaction of ketone silyl enol ethers with good control of both absolute and relative stereochemistry (Sch. 31) [90], Surprisingly, however, the aldol products were obtained in the silyl enol ether (ene product) form, with high syn diastereoselectivity from either geometrical isomer of the starting silyl enol ethers. 

Fused bicyclic morpholones 145 and 146 have been synthesized in good yields but with modest diastereoselectivity from 1,3-oxazolidines 144 via reaction of the nitrogen of the oxazolidine with a metallocarbene intermediate followed by a benzyl migration (Scheme 37) <2003TA917>. 

Lubineau reported a series of research works on the preparation of C-branched monosaccharides and C-disaccharides under indium promoted Barbier-type allylation in aqueous media [49]. In the case of substrate 97, the reaction, which took place in H20/Et0H (1 2) at 50 °C, gave unique stereoisomer 98 with complete regio- and diastereoselectivity. From... 

Simple diastereoselection from precursors that do not contain a chiral center is controlled by two factors ... 

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