To assign the relative stereochemistry as l or u, the
first step is to assign the absolute configuration at each stereocentre
as R or S. For compounds 4.1-4.4, these are given
in Table 4.1. For compound 4.1, both stereocentres have the
same absolute configuration (S), so this the l-diastereomer.
Similarly, for compound 4.2, both stereocentres have the same absolute
configuration (R), so this the l-diastereomer. For compound
4.3,
the two stereocentres have different absolute configurations (one R
and one S), so this is the u-diastereomer. Similarly, for
compound 4.4, the two stereocentres have different absolute configurations
(one R and one S), so
this is the u-diastereomer.
To assign the relative stereochemistry using the syn / anti nomenclature, the compounds must be drawn in flying wedge projections which requires rotation around the C2-C3 bond as shown below. Rotatable 3D structures of each compound (in the order 4.1-4.4) are also shown below. For compound 4.1, the two non-hydrogen substituents (the amino and hydroxyl groups shown in red below) are on opposite sides of the carbon chain, so this is the anti-diastereomer. Similarly, for compound 4.2, the two non-hydrogen substituents are on opposite sides of the carbon chain, so this is the anti-diastereomer. For compound 4.3, the two non-hydrogen substituents are on the same side of the carbon chain, so this is the syn-diastereomer. Similarly, for compound 4.4, the two non-hydrogen substituents are on the same side of the carbon chain, so this is the syn-diastereomer.
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To assign the relative stereochemistry as l or u, the first step is to assign the absolute configuration at each stereocentre as R or S. For compounds 4.5-4.7, these are given in Table 4.2. For compound 4.5, both stereocentres have the same absolute configuration (R), so this the l-diastereomer. Similarly, for compound 4.6, both stereocentres have the same absolute configuration (S), so this the l-diastereomer. For compound 4.7, the two stereocentres have different absolute configurations (one R and one S), so this is the u-diastereomer.
To assign the relative stereochemistry using the syn / anti nomenclature, the compounds must be drawn in flying wedge projections which requires rotation around the C2-C3 bond as shown below. Rotatable 3D structures of each compound are also shown below. For compound 4.5, the two non-hydrogen substituents (the two bromine atoms shown in red below) are on the same side of the carbon chain, so this is the syn-diastereomer. Similarly, for compound 4.6, the two non-hydrogen substituents are on the same side of the carbon chain, so this is the syn-diastereomer. For compound 4.7, the two non-hydrogen substituents are on opposite sides of the carbon chain, so this is the anti-diastereomer. Note that comparison of the answers to parts 4.1-4.4 with those to 4.5-4.7 shows that there is no correlation between the l / u and syn / anti nomenclatures.
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To assign the relative stereochemistry as l or u, the first step is to assign the absolute configuration at each stereocentre as R or S. For compounds 4.10 and 4.12-4.14, these are given in Table 4.3. For compound 4.10, the stereocentres at C2 and C3 have the same absolute configuration (R and r), whilst the stereocentres at C3 and C4 have opposite absolute configurations (r and S). Hence, this is the lu-diastereomer. Note, that the first stereochemical descriptor corresponds to the relative stereochemistry of the two lowest numbered stereocentres. For compound 4.12, there are only two stereocentres (C2 and C4) and these both have the same absolute configuration (R), so this the l-diastereomer. Similarly, for compound 4.13, there are only two stereocentres (C2 and C4) and these both have the same absolute configuration (S), so this the l-diastereomer. For compound 4.14, the stereocentres at C2 and C3 have opposite absolute configurations (R and s), whilst the stereocentres at C3 and C4 have the same absolute configuration (s and S). Hence, this is the ul-diastereomer.
To assign the relative stereochemistry using the syn / anti nomenclature, the compounds must be drawn in flying wedge projections as shown below. Rotatable 3D structures of each compound are also shown below. The non-hydrogen substituent attached to the lowest numbered stereocentre (C2) is then used as a reference to determine the relative stereochemistry. The reference substituent is shown in red in the diagrams below. For compound 4.10, the non-hydrogen substituent attached to C3 is on the opposite face of the molecule as the reference substituent on C2, whilst the non-hydrogen substituent on C4 is on the same face as the reference substituent on C2. Hence, this is the anti-syn diastereomer. For compound 4.12, the non-hydrogen substituent attached to C3 is on the opposite face of the molecule as the reference substituent on C2, as is the non-hydrogen substituent on C4. Hence, this is the anti-anti diastereomer. Similarly, for compound 4.13, the non-hydrogen substituent attached to C3 is on the opposite face of the molecule as the reference substituent on C2, as is the non-hydrogen substituent on C4. Hence, this is the anti-anti diastereomer. For compound 4.14, the non-hydrogen substituent attached to C3 is on the same face of the molecule as the reference substituent on C2, as is the non-hydrogen substituent on C4. Hence, this is the syn-syn diastereomer.
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