3-Chlorobutanal

Several disconnections are possible for this compound, of which (a) and (b) are fairly obvious. However, although the starting material for (a) could be obtained from sodium benzenethio-late and 3-chlorobutan-2-one, that for (b) is more difhcult as ortho alkylation of benzenethiol would be necessary ... 

The free radical bromination of optically active (R)-2-chlorobutane at C-3 produces the two diastereomers of 2-bromo-3-chlorobutane in unequal amounts, explained ... 

Amino acid-modified furocoumarins were prepared by condensation of the T-hydroxysuccinimide ester of 3-(2,3,5-trimethyl-7-oxofuro[3,2-g]chromen-6-yl)propanoic acid 51 with amino acids <2003JPH177>. Ethyl 3-(7-hydroxy-4-methyl-2-oxo-2/7-3-chromenyl)propanoate 49, prepared in 56% yield by Pechmann condensation of resorcinol and diethyl 2-acetylglutarate in the presence of HCl, was condensed with 3-chlorobutan-2-one in the presence of base to give propanoate 50 (Scheme 4). The MacLeod method was then used to fuse a furan ring to the benzopyran-2-one system. Heating 50 with NaOH solution (1 M) readily cyclized it into the corresponding psoralen furocoumarin 51 with simultaneous hydrolysis of the ester. 

Apart from geometric isomers, most other compounds that show diastereomerismhave two or more chirality centers, usually asymmetric carbon atoms. For example, 2-bromo-3-chlorobutane has two asymmetric carbon atoms, and it exists in two diastereomeric forms (shown next). Make molecular models of these two stereoisomers. 

Since these compounds are stereoisomers but not enantiomers, they must be diastere-omers. In fact, both of these diastereomers are chiral and each has an enantiomer. Thus, there is a total of four stereoisomeric 2-bromo-3-chlorobutanes two pairs of enantiomers. Either member of one pair of enantiomers is a diastereomer of either member of the other pair. 

In the preceding section, we saw there are four stereoisomers (two pairs of enantiomers) of 2-bromo-3-chlorobutane. These four isomers are simply all the permutations of (R) and (S) configurations at the two asymmetric carbon atoms, C2 and C3 ... 

What are the names of the compounds which are formed by the reaction of 2 - bromopropane and 2 - methyl - 3 - chlorobutane with a sufficient amount of Na ... 

Draw all the stereoisomers of each of the molecules (a)- e) assign configuration to stereogenic centres and say whether each stereoisomer is chiral (a) 2,3-dibromobutane (b) 2-bromo-3-chlorobutane (c) the monomethyl ester of tartaric acid (2,3-dihy-droxybutanedioic acid) (d) 2,3-difluoropentane (e) 1,3-dichloro-cyclopentane. 

Trimethylethylene dibromide has been obtained by adding bromine to trimethylethylene and to tertiary amyl alcohol. Methyl iso-propyl ketone has been prepared by heating trimethylethylene glycol (2-methyl-2,3-dihydroxybutane) with dilute hydrochloric acid, by heating trimethylethylene chloro-hydrin (2-methyl-2-hydroxy-3-chlorobutane) in a sealed tube with water, by boiling trimethylethylene dibromide with lead oxide (PbO) and water, by heating trimethylethylene dibromide with water, by the action of iso-propylmagnesium bromide on acetic anhydride, and by a variety of other methods not particularly related to the one here described. ... 

Neureiter and Bordwell46 report that addition of thioacetic acid to cis- and /r[Pg.604]

A-9. Two (2/ ,3 )-2-bromo-3-chlorobutane and (25, 35)-2-bromo-3-chlorobutane they are bias tereomers. 

When a molecule has more than one stereocenter, as 2-bromo-3-chlorobutane does, it will have more than two stereoisomers. In particular, n stereocenters give rise to as many as 2" stereoisomers. If we consider the case where n = 3, how are all the 2" = 8 stereoisomers related Because an object has only one mirror image, if we pick any one of these (stereoisomer A), it may have no more than one enantiomer (stereoisomer B). What about the other six isomers They are also stereoisomers of A, but they can t be its mirror images. The relationship of any of these other six molecules with A is described by a new term diastereomer. Diastereomers are stereoisomers that are not mirror images of each other. Because diastereomers are not mirror images of each other, they have different physical properties and can therefore be separated by standard laboratory techniques. This is very important. This feature distinguishes diastereomers from enantiomers, which cannot readily be separated from one another. 

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