Erythro-9-

It is found in lichens and in some algae. It has m.p. 120 C, is very soluble in water and is about twice as sweet as sucrose. It is a reference compound upon which the erythro nomenclature is based. 

Alkylation of aldol type educts, e.g., /3-hydroxy esters, using LDA and alkyl halides leads stereoselectively to erythro substitution. The erythro threo ratio of the products is of the order of 95 5. Allylic and benzylic bromides can also be used. The allyl groups can later be ozonolysed to gjve aldehydes, and many interesting oligofunctional products with two adjacent chiral centres become available from chiral aldol type educts (G. Prater, 1984 D. Seebach, 1984 see also M. Nakatsuka, 1990, p. 5586). 

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

Cram erythro-products" (G.E. Keck, 1984 A, B, C). [3-(Silyloxy)allyl]stannanes and O-pro-tected a- or y -hydroxy aldehydes yield 1,2,3- or 1,2,4-triols with three chiral centres with high regio- and diastereoselectivity (G.E. Keck, 1987). 

In stereoselective antitheses of chiral open-chain molecules transformations into cyclic precursors should be tried. The erythro-configurated acetylenic alcohol given below, for example, is disconnected into an acetylene monoanion and a symmetrical oxirane (M. A. Adams, 1979). Since nucleophilic substitution occurs with inversion of configuration this oxirane must be trens-conilgurated its precursor is commercially available trans-2-butene. 

Organic chemisfs use an informal nomenclafure sysfem based on Eischer projec lions fo dislmguish belween diasfereomers When fhe carbon chain is vertical and like subsliluenls are on fhe same side of fhe Eischer projecfion fhe molecule is described as fhe erythro diaslereomer When like subsliluenls are on opposite sides of fhe Eischer projecfion fhe molecule is described as fhe threo diaslereomer Thus as seen m fhe... 

Erythro and three describe the relative configuration (Section 7 5) of two chirality centers within a single molecule... 

Fischer projections of the stereoisomeric 2 3 dihydroxybutanoic acids compounds I and II are erythro stereoisomers and III and IV are threo... 

Draw Fischer projections or make molecular models of the four stereoisomeric 3 ammo 2 butanols and label each erythro or threo as appropriate... 

Stereoisomers that are not enantiomers are classified as diastereomers Each enantiomer of erythro 3 bromo 2 butanol is a diastereomer of each enantiomer of threo 3 bromo 2 butanol... 

The two stereoisomeric furanose forms of d erythrose are named a d erythro furanose and p d erythrofuranose The prefixes a and p describe fhe relative configu ration of fhe anomeric carbon The configurafion of fhe anomeric carbon is compared wifh fhaf of fhe highesf numbered chiralify cenfer m fhe molecule—fhe one fhaf defer mines whefher fhe carbohydrafe is d or l Chemisfs use a simplified informal version of fhe lUPAC rules for assigning a and p fhaf holds for carbohydrates up fo and mclud mg hexoses... 

Erythro (Section 7 11) Term applied to the relative configura tion of two chirality centers within a molecule The erythro stereoisomer has like substituents on the same side of a Fischer projection... 

---