Predict the number of stereoisomers in ${\mathrm{CH}}_2\mathrm{OH}\cdot (\mathrm{CHOH}{)}_4\cdot \mathrm{CHO}$

Terminal groups are different. The number of asymmetric C atoms is four.

$\left({\mathrm{HOCH}}_2^{ }\stackrel{*}{\mathrm{C}}\mathrm{H}\left(\mathrm{OH}\right)\stackrel{*}{\mathrm{C}}\mathrm{H}\left(\mathrm{OH}\right)\stackrel{*}{\mathrm{C}}\mathrm{H}\left(\mathrm{OH}\right)\stackrel{*}{\mathrm{CH}}\left(\mathrm{OH}\right){\mathrm{CH}}_2\mathrm{OH}\right)$

Therefore, the number of stereoisomers (optical isomers) is

$2^n=2^4=16$

(where r is the number of asymmetrical C atoms)