Threo selectivity

Given the relatively rare appearance of oxetanes in natural products, the more powerful functionality of the Patemo-Biichi reaction is the ability to set the relative stereochemistry of multiple centers by cracking or otherwise derivitizing the oxetane ring. Schreiber noted that Patemo—Btlchi reactions of furans with aldehydes followed by acidic hydrolysis generated product 37, tantamount to a threo selective Aldol reaction. This process is referred to as photochemical Aldolization . Schreiber uses this selectivity to establish the absolute stereochemistry of the fused tetrahydrofuran core 44 of the natural product asteltoxin. ... 

On the other hand, if the allylsilane anion is first complexed with certain metals, a-regioselectivity then predominates, and a high degree of complementary diastereoselectjvity (19) can be attained with aldehydes as electrophiles. For example, boron, aluminium and titanium complexation all induce threo selectivity whereas the use of tin results in an erytbro... 

L2909>. An organocatalytic addition of 2-trimethylsilyloxyfuran to aldehydes using 10 mol% of l,3-bis(3-(trifluoromethyl)phenyl)urea provided adducts with modest threo selectivity <06TL8507>. A syn-selective, enantioselective, organocatalytic vinylogous Mukaiyama-Michael addition of 2-trimethylsilyloxyfuran to (E)-3-... 

While reaction of the acetate 40 as well as the acetyl- and phthalimide derivatives of chiral amine (41b and 41c) proceeded with erythro diastereoselectivity (in accordance with the classical cis effect, minimization of 1,3-allyhc strain) (Table 6, entries 8, 10, 11), for the allylic alcohols 39, primary allyhc amine 41a, silyl enol ethers 42 and enol ether 43 threo selectivity was observed (Table 6, entries 1-7, 9, 12-14) (see also Scheme 24). For allyhc alcohols with an alkyl group R cis to the substituent carrying the hydroxyl group, diastereoselectivity was high (Table 6, entries 1-7) in contrast, stereoselection was low for allylic alcohols which lack such an R cis) substituent (substrates 39h and 39i, see Figure 4). 

This problem was solved by Adam and coworkers in 1994-1998. They presented a high-yielding and diastereoselective method for the preparation of epoxydiols starting from enantiomerically pure allyhc alcohols 39 (Scheme 69). Photooxygenation of the latter produces unsaturated a-hydroxyhydroperoxides 146 via Schenck ene reaction. In this reaction the (Z)-allylic alcohols afford the (5, 5 )-hydroperoxy alcohols 146 as the main diastereomer in a high threo selectivity (dr >92 8) as racemic mixmre. The ( )-allylic alcohols react totally unselectively threolerythro 1/1). Subsequent enzymatic kinetic resolution of rac-146 threolerythro mixture) with horseradish peroxidase (HRP) led to optically active hydroperoxy alcohols S,S) and (//,5 )-146 ee >99%) and the... 

In the epoxidation of acyclic allylic alcohols (Scheme 6), the diastereoselectivity depends significantly on the substitution pattern of the substrate. The control of the threo selectivity is subject to the hydroxyl-group directivity, in which conformational preference on account of the steric interactions and the hydrogen bonding between the dioxirane oxygen atoms and the hydroxy functionality of the allylic substrate steer the favored 7r-facial... 

Radical cylization of a P-alkoxymethacrylate leads to the stereoselective preparation of the benzylether of (+)-methyl nonactate, demonstrating 2J5-cis selectivity in the radical cydization step and threo selectivity in the hydrogen abstraction step <990L1127>. 

When racemic methyl a-(l-hydroxyethyl)aciylate is hydrogenated by using the (S)-BINAP-Ru catalyst, the R substrate is depleted more easily than the S. At 76% conversion, the unreacted S enantiomer is obtained in greater than 99% ee, as well as a 49 1 mixture of the threo (2R,3R) and the erythro saturated products. Hydrogenation of the S substrate with either antipodal Ru catalyst results in 2S,3S hydroxy ester with equally high threo selection (>23 1). These data indicate operation of overwhelming substrate control in this particular reaction. 

THREO-SELECTIVE REDUCTION OF (J-KETO AMIDES WITH PhMe2SiH/F- REAGENT a... 

Homoallylic alcohols (8, 111-112). The high rltreo-selectivity observed in the reaction of benzaldehyde with crotyl bromide (either irons or cis) is general for relatively unhindered aldehydes (equation I). High threo selectivity is still observed in the reaction with an a-methyl substituted aldehyde, but a-asymmetric induction (at C3) is rather low (2.2 1) with simple aldehydes (equation II).1... 

The a-C-H hydroxyalkylation of THF (1) with aldehydes provides a-alkylated products in comparable yields under triethylborane-air (method A) or triethylbor-ane-TBHP (method B) conditions (Table 1). The ease of operation and the relatively mild conditions are the merits of the former, whereas a short reaction time makes the latter highly efficient. Table 1 shows the general applicability of the two methods to various aldehydes. The yield and threo selectivity of products 3 4 are generally high for all aromatic aldehydes except ortho-substituted benzaldehyde (entry 4) and moderate to low for aliphatic substrates (entries 5 and 6). 

Photocycloaddition reactions of aromatic aldehydes with cyclic ketene silylacetates have been investigated by Abe and coworkers [61]. Regio- and diastereoselective formation of the bicyclic 2-alkoxyoxetanes 69 was observed in high yields. Hydrolysis of these acid-labile cycloadducts with neutral water efficiently gave aldol-type adducts 70 with high threo-selectivity (Sch. 18). 

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