Monday, March 25, 2024

α and β again

The descriptors ‘α’ and ‘β’ are also used in carbohydrate nomenclature to specify configuration of cyclic monosaccharides [1, P-102.3.4.2.1]. You may remember that aldehydo-glucose, the open-chain form of glucose, has four chiral centres. Consider the structures (a) and (b):

(a) (b)
  1. aldehydo-D-gluco-hexose (carbohydrate)
    aldehydo-D-glucose (carbohydrate)
    (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanal (substitutive)
  2. aldehydo-L-gluco-hexose (carbohydrate)
    aldehydo-L-glucose (carbohydrate)
    (2S,3R,4S,5S)-2,3,4,5,6-pentahydroxyhexanal (substitutive)

Upon cyclisation of either enantiomer, an extra chiral centre is created at the position 1. This centre is referred to as anomeric centre [2, 2-Carb-6.1] and two resulting stereoisomers are anomers. For example, cyclisation of aldehydo-D-glucose (a) brings about two major forms of D-glucose, (c) and (d):

(c) (d)

Sunday, March 10, 2024

α, β, ξ

Here’s a molecule everybody must have heard about: testosterone (a).

(a)
  1. testosterone (INN)
    17β-hydroxyandrost-4-en-3-one (fundamental parent + substitutive)
    (1S,3aS,3bR,9aR,9bS,11aS)-1-hydroxy-9a,11a-dimethyl-1,2,3,3a,3b,4,5,8,9,9a,9b,10,11,11a-tetradecahydro-7H-cyclopenta[a]phenanthren-7-one (fused ring + additive + substitutive)

Monday, February 12, 2024

Planar chirality

In most organic chemistry textbooks, double bond cis/trans isomerism is exemplified by alkenes. It is also observed in cycloalkenes such as cyclooctene that can exist as either cis (a) or trans (b) isomer:

(a) (b)
  1. (Z)-cyclooctene (PIN)
    cis-cyclooctene
  2. (E)-cyclooctene (PIN)
    trans-cyclooctene

To the trans isomer, there is a twist — and the pun is fully intended. Have a look at the structures (c) and (d) (or at their 3-D models here, Fig. 2 and Fig. 3, respectively).

(c) (d)
  1. (1E,1P)-cyclooct-1-ene (PIN)
    (E,P)-cyclooctene
    (E,Rp)-cyclooctene
  2. (1E,1M)-cyclooct-1-ene (PIN)
    (E,M)-cyclooctene
    (E,Sp)-cyclooctene

Sunday, December 10, 2023

Axial chirality

Have a look at the structures (a) and (b). They are the stereoisomers of laballenic acid, with (a) is naturally occurring in plants of the Lamiaceae family. What kind of stereoisomers are they?

(a) (b)
  1. (−)-laballenic acid (trivial)
    (5M)-octadeca-5,6-dienoic acid (substitutive, PIN)
    (5Ra)-octadeca-5,6-dienoic acid (substitutive)
  2. (+)-laballenic acid (trivial)
    (5P)-octadeca-5,6-dienoic acid (substitutive, PIN)
    (5Sa)-octadeca-5,6-dienoic acid (substitutive)

If there was just one double bond in the middle of the molecule, we’ll be dealing with cis/trans isomerism. But we have two cumulative double bonds, which makes our molecules chiral, even though there are no chiral atoms. Why?

Wednesday, September 27, 2023

cis and trans

What’s the difference between the structures (a) and (b)?

(a) (b)
  1. (2Z)-but-2-ene (PIN)
    cis-but-2-ene
  2. (2E)-but-2-ene (PIN)
    trans-but-2-ene

Wednesday, September 13, 2023

Enantiomers

Have a look at the structures (a) and (b).

(a) (b)
  1. (+)-amphetamine (trivial)
    d-amphetamine (trivial)
    dextroamphetamine (trivial)
    dexamfetamine (INN)
    (2S)-1-phenylpropan-2-amine (substitutive)
  2. (−)-amphetamine (trivial)
    l-amphetamine (trivial)
    levoamphetamine (trivial)
    levamfetamine (INN)
    (2R)-1-phenylpropan-2-amine (substitutive)

Monday, August 07, 2023

Polyhedral symbols and configuration indices

Although structural descriptors such as we’ve seen in the names of boron hydrides, for example catena or closo, provide information on atomic connectivity, they tell us little or nothing about the geometry of the molecule.

Have a look at the structures (a) and (b):

(a) (b)
  1. (SPY-5)-pentaoxidotungstate(4−) (additive)
  2. (TBPY-5)-pentaoxidotungstate(4−) (additive)

Both of them can be named additively pentaoxidotungstate(4−). Yet, as you can see, they have very different shapes.