1.3- dioxolane, basicity

Chemicals responsible for odor in some PUR foams were synthesised by polymerisation of PO in CH2CI2 with Bp2(C2H )20 catalyst (114). The yield was 25% volatile material and 75% polymeric material. The 25% fraction consisted of dimethyldioxane isomers, dioxolane isomers, DPG, TPG, crown ethers, tetramers, pentamers, etc, and 2-ethy1-4,7-dimethyl-1,3,6-trioxacane (acetal of DPG and propionaldehyde). The latter compound is mainly responsible for the musty odor found in some PUR foams. This material is not formed under basic conditions but probably arises during the workup when acidic clays are used for catalyst removal. 

Dioxolanes haye been prepared from a carbonyl compound and an epoxide (e.g., ketone/SnC, CCI4, 20°, 4 h, 53% yield or aldehyde/ Et4N Br, 125-220°, 2-4 h, 20-85% yield ). Perhalo ketones can be protected by reaction with ethylene chlorohydrin under basic conditions (K2CO3, pentane, 25°, 2 h, 85% yield or NaOH, EtOH—H2O, 95% yield ). 

When the carbonyl group is very electron-deficient, thus stabilizing the hemiacetal, a dioxolane can be prepared under basic conditions. 

It is stated in the basic patent that ethylene oxide (II) and 1.3-dioxolane (III) are the preferred materials. By the occasional incorporation of molecules containing two successive methylene groups the tendency of the molecules to unzip is markedly reduced. 

The initial investigation focused on the use of threitol-derived auxiliaries with various substituent groups on the dioxolane ring (Table 3.3). However, it became evident that the oxygen atoms in the substituents had a detrimental effect on selectivity. Comparison of the diastereoselectivities for the ketals 69-71, which contain Lewis basic sites in the substituents at the 1 and 2 positions, with those from simpler diol derived ketals 72-74 demonstrates the conflicting effects of numerous coordination sites. The simpler, diol-derived ketals provide superior results compared to the threitol derived ketals. The highest diastereoselectivity is observed in the case of the 1,2-diphenyl ethane-1,2-diol derived ketal 74. 

Certain cyclic ethers which will not homopolymerize will copolymerize with THF (25, 52). These cyclic ethers are stable five and six-membered ring compounds such as 2-methyltetrahydrofuran (2-MeTHF), and 1,4-dioxane (DOX). It is probable that 4-phenyl-l,3-dioxane (PhDOX), tetrahydropyran (THP), and 4-methyl-l,3-dioxolane (MDOL) which do not homopolymerize but which have been reported to copolymerize with BCMO (107, 108) would also copolymerize with THF. In the copolymerizations a correlation was again found between the basicity of the attacking ether and its reactivity with the cyclic oxonium ion. The... 

Acid catalysis is the common feature that links all of the methods of acetal formation discussed so far. Dioxolanation has also be accomplished under basic conditions using 2-bromoethanol82 as shown in the a-keto amide protection step [Scheme 2.38] taken from a synthesis of Strychnine.83 Success here is probably a consequence of the high electrophilicity of the a-keto amide ... 

Methylthiomethyl ethers are comparable in stability to alkoxymethyl ethers towards strongly basic conditions or mild acid. For example, methylthiomethyl ethers survive aqueous acetic acid under conditions that hydrolyse TBS ethers, dioxolanes or tetrahydropyranyl protectors. They can be removed under rather specific and mild conditions that do not affect most acetal-type protectors such as MEM. MOM. etc. (see above). The presence of the sulfur makes this group liable to oxidation by strong oxidants such as peracids. Cr(VI), and N-bromo-succinimide, and it will poison Pd catalysts. The methylthiomethyl group was First introduced by Corey and co-workers586 and its virtues exploited by them in a synthesis of the antibiotics Erythronolide587 588 and Brefeldin.589... 

Diols react with 1,2-dihalo compounds under basic conditions to form 1,3-dioxolanes. Treatment of 2-chloroethyl hemiacetals with base is an alternative approach to these cyclic acetals, " Similarly, 2-chlorocthyl ester enolatcs cyclizc readily.2-Hydroxy- " 2-phenoxy-, " and 2-alkoxy-l,3-dioxolanes are synthesized from epoxides "" or.vr/ -tet-raphenylethylene dichloride " using trifluoroacetic acid. " its silver salt, " or alkyl trifluoro-acetates. ... 

Computational studies concerning theoretical approaches to the intrinsic basicity of neutral nitrogen bases have been reported, including those of phos-phoranimines. The non-ionic phosphazene bases BEMP (112), BTPP (113) and (114, R = Ph) appear to be excellent catalysts for the Michael addition reactions. Thus the yield of the coupling reaction of ethyl isocyanoacetate with l,2-bis(4-bromomethylphenyl)ethane is increased by the addition of the phosphazene base BEMP. Polymer-supported BEMP (P-BEMP) has been applied for the allylation of 2H-benzo[d]l,3-dioxolan-5-ol by allyl bromide. " Cyclodehydration of 1,2 diacylhydrazines by tosyl chloride in the presence of P-BEMP leads to excellent yields of 1,3,4,-oxadiazoles. Addition of P-BEMP also improves the yield of the Hofmann elimination step in the synthesis of tertiary mines using REM resin (polymer-bound acrylate ester). ... 

The carbonyl protecting group in 22 is known as a dioxolane, a cyclic acetal. These acetals are more stable towards hydrolysis than acyclic ones, and therefore require treatment with pyridinium / -toluene-sulfonate (PPTS) under reflux. In general, acetals can only be cleaved using acidic conditions and are stable under basic conditions. 

Synthesis of model compounds and structural units are being investigated. A double Simmons-Smith reaction on the l,3-dioxolane-4,5-diylbis(alkene) 107 afforded the product 108 with excellent stereoselectivity. The required asymmetry in the double cyclopropanation was the result of coordination of the zinc carbenoid reagent by the Lewis basic dioxolane ring oxygen prior to each cyclopropanation event. The cyclopropanated product was converted to ( )-l,2-bis[(l 5,25)-2-methylcyclopropyl]ethene, a relevant model for the complete structural assignment of FR-900848. 

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