  Determining the level of chemical complexity within dense starless and
gravitationally bound prestellar cores is crucial for constructing chemical
models, which subsequently constrain the initial chemical conditions of star
formation. We have searched for complex organic molecules (COMs) in the young
starless core L1521E, and report the first clear detection of dimethyl ether
(CH$_3$OCH$_3$), methyl formate (HCOOCH$_3$), and vinyl cyanide (CH$_2$CHCN).
Eight transitions of acetaldehyde (CH$_3$CHO) were also detected, five of which
(A states) were used to determine an excitation temperature to then calculate
column densities for the other oxygen-bearing COMs. If source size was not
taken into account (i.e., if filling fraction was assumed to be one), column
density was underestimated, and thus we stress the need for higher resolution
mapping data. We calculated L1521E COM abundances and compared them to other
stages of low-mass star formation, also finding similarities to other
starless/prestellar cores, suggesting related chemical evolution. The scenario
that assumes formation of COMs in gas-phase reactions between precursors formed
on grains and then ejected to the cold gas via reactive desorption was tested
and was unable to reproduce observed COM abundances, with the exception of
CH$_3$CHO. These results suggest that COMs observed in cold gas are formed not
by gas-phase reactions alone, but also through surface reactions on
interstellar grains. Our observations present a new, unique challenge for
existing theoretical astrochemical models.
