S-Acetals

Before we leave the subject of SS-acetal cleavage, we should record a few ex-amples where proximate hydroxyl or ether functions greatly facilitate the dea- 

Acetals and hemiacetals imbedded in 5- and 6-membered rings are remarkably stable and their deliberate conversion to the acyclic carbonyl derivatives can be a problem. The high thermodynamic and acid stability of the corresponding 5,5-acetals can be used to liberate a carbonyl by using a 2-stage transprotection protocol. During a synthesis of the macrolide antibiotic Erythronolide B, Martin 

Like the 1,3-dithianes, 1,3-oxathianes can be metallated with alkyllithium reagents and they hydrolyse about 10,000 times faster with protic acids. However, their value is diminished by the limited stability of the lithio derivatives and their inherent lack of symmetry which introduces the complications of diaster-eoisomerism. Eliel and co-workers have exploited the diastereoisomerism of 

The next carbonyl protecting group stems from an attempt to retain the carba-nion-stabilising properties of the dithiane 45-acetal whilst increasing its hydro- 

Some of the other reactions in which N. 0-acetals have been used as chiral auxiliaries include conjugate addition reactions,172 eyelopropanation,211 addition of nucleophiles to cyclic ketones212 and Diels-Alder reactions213 

The greater part of this chapter is devoted to the metallation of S,S-acetals and some related compounds and subsequent reactions of the organometallic intermediates with a number of electrophilic reagents. Representative conditions for the metallation of saturated sulfur compounds are given in Table 5. 

Arens JF, Froling M, Froling A (1959) Reel Trav Chim Pays-Bas 78 663 

Shiner CS, Tsunoda T, Goodman BA, Ingham S, LeeS, Vorndam PE (1989) J AmChemSoc 111 1381 

Most of the syntheses with S,S-acetals published after the first review of Seebach [ 1] have been carried out with 1,3-dithianes as starting compound. Butyllithium has been commonly used as metallating agent in a solvent mixture of THF and hexane. 

In many cases the more easily and more cheaply available open-chain S,S-acetals, such as H2C(SCH3)2 and H2C(SC2H5)2, can be used as starting compounds instead of 1,3-dithianes. We found that the abstractions of a methylene proton from 1,3-dithiane and H2C(SCH3)2 or H2C(SC2Hs)2 by BuLi and the subsequent reactions with butyl bromide proceeded with comparable rates. Differences arise, however, when the alkyl derivatives have to belithiated. Whereas the lithiation of 2-alkyl-1,3-dithianes by BuLi seems to proceed cleanly [1], the open-chain derivatives 

An important issue in Schreiber s synthesis of of protecting group to mask the lactone until was protected in its lower oxidation state as treatment of the anomeric mixture of acetals lane, zinc iodide, and tetrabutylammonium anomeric 0,5-acetals that were separated by Towards the end of the synthesis, the lactone lysed hydrolysis of the 0,5-acetal in lOU to tion to lactone 101.4 in 77% yield for the two 

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A review discusses the condensation of aldehydes and ketones with glycerol to give 1,3-dioxanes and 1,3-dioxolanes. The chemistry of 0 0 and 0 S acetals has been reviewed, and a recent monograph discusses this area of protective groups in a didactic sense. ... 

The N,0- and N,S-heterocyclic fused ring products 47 were also synthesized under radical chain conditions (Reaction 53). Ketene acetals 46 readily underwent stereocontrolled aryl radical cyclizations on treatment with (TMSlsSiH under standard conditions to afford the central six-membered rings.The tertiary N,0- and N,S-radicals formed on aryl radical reaction at the ketene-N,X(X = O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from (TMSlsSiH was investigated and found to provide higher selectivities than BusSnH. 

Pulsed current experiments of aqueous acetate solutions indicate that at least in aqueous solution a platinum oxide layer seems to be prerequisite for the da arboxy-lation to occur. Only at longer pulse durations (> 10 s) is ethane produced [73,74]. These are times known to be necessary for the formation of an oxide film. At a shorter pulse length (<10"" s) acetate is completely oxidized to carbon dioxide and water possibly at a bare platinum surface [75]. The potent dynamic response in the electrolysis of potassium acetate in aqueous solution also points to an oxide layer, whose... 

Ci jHijBrjNO 4614-45-3) see Oxcarbazepine 3-acetyl-2,5-dichlorothiophene (CSH4CI2OS 36157-40-1) see Brinzolamide A -acetyl-2, 3 -didehydro-2, 3 -dideoxycytidine S -acetate (CJ3H15N3O5 62805-52-1) see Zalcitabine... 

The penicillin-N,S-acetal 479 reacts with N,N-bis(trimethylsilyl)formamide 22c and Hg(OAc)2, apparently via the iminium salt 480, to give the penicillin-N,N-acetal 481 in 65% yield [64]. On treatment of racemic y-ketoesters such as 482 with chiral silylated 1,3-mercaptoalcohols such as 483, in the presence of TMSOTf 20, at room temperature a kinetically controlled 2 1 mixture of the 0,S-acetals... 

Finally, the silylated 0,S-acetal of formaldehyde Me3SiOCH2SMe 1275 is formed as intermediate in the treatment of alcohols or glycols with DM SO and TCS 14 the final products are 0,0-acetals such as ( -BuO)2CH2 1280 (cf. Section 8.2.1). 

Phenylpropanal 583 reacts with phenylthiotrimethylsilane 584 in the presence of TiCl4, via the 0,S-acetal 585, to give the S,S-acetal 586 [144]. Conducting the reaction in the presence of allyltrimethylsilane 82 and SnCl furnishes the allylic sulfides 587 and 588 in 3 1 ratio and 56% yield [144] (Scheme 5.45). 

On reacting aldehydes such as propionic aldehyde, however, with a 1 1-mixture of a silylated allyl- or benzyl alcohol such as 589 and phenylthiotrimethylsilane 584 in the presence of TMSOTf 20 0,S-acetals such as 591 are obtained, via the probable intermediate 590, in high yields [63]. The a-ketoamide 592 is converted by methylthiotrimethylsilane 593/F3B-OEt2 into the bisthioketal 594 in 81% yield [145]. Ethylenethioketals such as 595 are cleaved by MesSiBr 16 or Me3SiI 17 in... 

SO acetate = l-S acetic acid Now substitute and calculate the required... 

The a-oxoketene dithioacetals 6 (6.1-6.43) employed in this work as three carbon 1,3-biselectrophilic components have been drawn from various active methylene ketones and are described in the Table-1. The corresponding a-oxoketene N,S- and 0,S-acetals (17.1- 17.15) which are usually derived from a-oxoketene dithioacetals are described in Table-2. These examples are only a representative groups to demonstrate the general application of the new Jl-heteroaromatic annulation methodology... 

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