Hydroxy selective protection

Methylation of avermectins B and B2 leads to the corresponding derivatives of the A series (49). A procedure involving the oxidation of the 5-methoxy group with mercuric acetate and NaBH reduction of the 5-keto-intermediate allows the conversion of the A to the B components (50). The 23-hydroxy group of the "2" components, after selective protection of the other secondary hydroxy groups, is converted to a thionocarbonate, which can be elirninated to give the 22,23-double bond of the "1" components alternatively it can be reduced with tributyltin hydride to the 22,23-dihydro derivatives (= ivermectins) (51). 

Oxazolidines are prepared to allow selective protection of the a- or co-COjU groups in aspartic and glutamic acids and a-hydroxy acids. 

As they are available from natural sources in enantiomerically pure form, carbohydrates are useful starting materials for syntheses of enantiomerically pure compounds. However, the multiple hydroxy groups require versatile methods for selective protection, reaction, and deprotection. Show how appropriate manipulation of protecting groups and/or selective reagents could be used to effect the following transformations. 

The five-membered 1,2-cyclic carbonate was isolated as the only product (regio-selective protection of the vicinal diol system).[255J Analogous formation of a cyclic carbonate containing a secondary hydroxy group is described in reference [256]. 

SELECTIVE PROTECTION OF 1,3-DIOLS AT THE MORE HINDERED HYDROXY GROUP 3-(METHOXYMETHOXY) 1-BUTANOL (1-Butanol, 3-(methoxymethoxy)-)... 

The transformation of L-arabinose (58) to lactone 57 was based on a route developed by Marquez and Sharma [51] Selective protection of the primary hydroxy group with TBDPSCl and oxidation of the lactol moiety with bromine afforded lactone 59. Subsequent selective deoxygenation a to the carbonyl group proceeded under Barton-McCombie conditions providing lactone 57 in 21% yield (Scheme 14). 

The synthesis of TGS (2) involves a series of selective protection and deprotection steps so that the 4-hydroxy group can be converted to chloro with inversion of configuration.23-25 Differentiation between the primary hydroxy groups of the two sugar moieties is also required (Scheme 4.1).26-27... 

The level of selectivity that can be achieved in the formation of trimethylsilyl ethers is illustrated by the selective protection of the hydroxy group at C-ll in the methyl ester of the prostaglandin 15-methyl PGF20 (Equation Si2.2). Thus the less sterically hindered secondary alcohol at C-ll is selectively protected in the presence of the more sterically hindered secondary alcohol at C-9 and the tertiary alcohol at C-15. 

Amino-type groups are also present in carbohydrate molecules although the number is usually much fewer than that of the hydroxy groups. Selective protections among the amino groups are therefore less frequent. Because of their stronger nucleophilicity,... 

Androstenolone, 1, can be transformed microbiologically to the 7a,15a-dihy-droxy derivative 2 by the action of Colletotrichium Uni. During formation of the acetal (3), inversion takes place on C-7. Acidic cleavage of 3 results in the triol, 4, which can also be produced by direct acidic catalysis from 2 [12,13]. After selective protection to the 3/l,15a-dipivalate (5), the 15/1,16/1-methylene compound, 6, can be synthetized, and then stereoselectively transformed to the 5/ ,6/ -epoxide, 7. This reacts with triphenylphosphine and tetrachloromethane in pyridine to produce the 7a-chloro derivative, 8. On treatment with zinc and acetic acid, 8 can be converted to the key compound 9, which has a 5/i-hydroxy-6-ene structure. Compound 9 can then be methylenated stereoselectively in the 6/1,7/1 position by the Simmons-Smith method. The last three steps - 10 —> 11 —> 12 — drospirenone -include the build-up of the spironolactone ring, after which water is lost from the molecule and oxidation affords drospirenone. 

The primary hydroxy group has to be selectively protected. A silyl protecting group is introduced. 

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