Let $f$ be a bounded tempered distribution, that is, $f\ast\varphi \in L^\infty(\mathbb R^n) $ for every Schwartz function $\varphi$. If $g \in L^1(\mathbb R^n)$, does the following definition define a tempered distribution:
$$ \langle f \ast g,\varphi \rangle = \int\limits_{{\mathbb R^n}} {(f \ast \tilde \varphi } )(x)\tilde g(x) \, dx$$
where $\varphi$ is a Schwartz function and $\tilde \varphi(x)=\varphi(-x)$?
Yes. Take a look on this book in the part of tempered distributions:
"Michael Eugene Taylor - Partial Differential Equations Volume I Basic Theory"
Ok lets try. Let $S$ be the space of Schwartz functions.
Note first that this number is well defined:
\begin{eqnarray} |\langle f\ast g,\phi\rangle| &\leq& \int_{\mathbb{R}^{n}}|f\ast \tilde{\phi}||\tilde{g}| \nonumber \\ &\leq& \|f\ast \tilde{\phi}\|_{\infty}\|\tilde{g}\|_{1} \nonumber \end{eqnarray}
On the other hand, as you can see in that book, $f\ast \tilde{\phi}$ is a tempered distribution, so its is continuous i.e. $$(\forall\phi)\Bigl(\phi\in S\Rightarrow |f\ast \tilde{\phi}|\leq Cp_{k}(\tilde{\phi})\Bigr)$$ where $p_{k}(\phi)$ is defined as there "in the book".
Now
\begin{eqnarray} |\langle f\ast g,\phi\rangle| &\leq& \int_{\mathbb{R}^{n}}|f\ast \tilde{\phi}||\tilde{g}| \nonumber \\ &\leq& Cp_{k}(\tilde{\phi})\|\tilde{g}\|_{1} \nonumber \end{eqnarray}
With the last inequality you can conclude.