Page 148 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

128 CHO OH CH O
H OH HO H
CHAPTER 2 O
H OH HO
Stereochemistry, CH OH OH H H OH
2
Conformation, 2 CH OH
and Stereoselectivity anti 2R,3R
2R,3R
(D-erythrose)
OH
CHO O CH O
HO H HO H OH
HO H OH H HO H
OH CH OH
CH 2 anti 2S,3R 2
2S,3S
(L-erythrose)
OH
CH O
CHO O HO H
HO H HO
H OH OH H
HO H
CH 2 OH syn 2S,3R CH 2 OH
2S,3R
(D-threose)
OH
CH O
CHO O H
H OH HO OH
HO H OH H
H OH
2
CH 2 OH syn 2R,3S CH OH
2R,3S
(L-threose)
Fig. 2.5. Fischer, extended, and Newman projection representations of
the stereoisomers of 2,3,4-trihydroxybutanal.

by Newman projection formulas. Figure 2.5 shows 2,3,4-trihydroxybutanal (now also
with its carbohydrate names, erythrose and threose) as Fischer projection formulas as
well as extended and Newman representations.
Because the Fischer projection formulas represent an eclipsed conformation of the
carbon chain, the relative orientation of two adjacent substituents is opposite from the
extended staggered representation. Adjacent substituents that are anti in an extended
representation are on the same side of a Fischer projection formula, whereas adjacent
substituents that are syn in an extended representation are on opposite sides in a
Fischer projection. As with extended representations, an enantiomeric pair represented
by Fischer projection formulas has the opposite configuration at all stereogenic centers
(depicted as left or right.)

2.1.5. Other Types of Stereogenic Centers
Although asymmetrically substituted carbon atoms are by far the most common
type of stereogenic center in organic compounds, several other kinds of stere-
ogenic centers are encountered. Tetravalent nitrogen (ammonium) and phosphorus
(phosphonium) ions are obvious extensions. Phosphine oxides are also tetrahedral and
are chiral if all three substituents (in addition to the oxygen) are different. Not quite